diff --git a/REUSE.md b/REUSE.md
index cf130ce..13e2b1f 100644
--- a/REUSE.md
+++ b/REUSE.md
@@ -15,13 +15,14 @@
 | `SudoHostOps.run` | internal/storage/hostops.go | `run(ctx, name, args...) error` | allowlisted exec with stderr-wrapped error | Every arg pre-validated via validate.go before this is called |
 | `Prober.Probe` | internal/capability/probe.go | `Probe(ctx) []Status` | live sudo-policy capability check (`sudo -n -l --`) | Needs a DIRECT runner (never the sudo-prefixing one — double-sudo); never executes probed cmds. v0.86.0: config-gated caps (`Capability.GatedBy` + `Prober.GateActive`) report `inactive`/"disabled by configuration" ONLY when healthy — broken plumbing stays degraded; the pbsdr-* gate answers from `pbsdr.Manager.DRConfigured` (marker-backed across restarts) |
 | ~~`stageTemp`~~ (REMOVED v0.146.0, R-861) | — | — | — | Nothing the agent writes is `install`ed where root reads it any more: use `felhom-priv-apply` (below) or ship a fixed file in the bundle |
-| `felhom-priv-apply` (v0.146.0, R-861) | configs/felhom-priv-apply | `felhom-priv-apply unit <name> \| dnsmasq <tmp> <name> \| wg \| sshd-config \| sshd-key \| controller-image <vmid>` (the last reads the ref on stdin, R-861 (a) A1) | ANY agent-rendered file a root program reads (systemd unit, dnsmasq drop-in, wg-quick conf, OOB sshd) — fixed source + destination, CONTENT checked against the agent's own renderers | A new renderer needs a verb + a contract test (`internal/privapplytest.Check`) feeding its REAL output; never a new `install` sudoers line |
+| `felhom-priv-apply` (v0.146.0, R-861) | configs/felhom-priv-apply | `felhom-priv-apply unit <name> \| unit-forget <name> (v0.159.0, R-1124) \| dnsmasq <tmp> <name> \| wg \| sshd-config \| sshd-key \| controller-image <vmid>` (the last reads the ref on stdin, R-861 (a) A1) | ANY agent-rendered file a root program reads (systemd unit, dnsmasq drop-in, wg-quick conf, OOB sshd) — fixed source + destination, CONTENT checked against the agent's own renderers | A new renderer needs a verb + a contract test (`internal/privapplytest.Check`) feeding its REAL output; never a new `install` sudoers line |
 | `privapplytest.Check` | internal/privapplytest/check.go | `Check(t, verb, name, content) string` | the Go↔root-checker contract: a renderer's real output must read `OK` | Skips without python3; one call per rendered shape + one refused control |
 | `BUNDLE_FILES` + `Bundle` (mode `bundle`, `--install-bundle`; mode `agent_update` v0.146.0, R-861) | configs/felhom-os-apply | the ONE table of root-owned paths + the installer of them | ANY new root-owned file the installer writes (sudoers line, wrapper, unit) — add it to the table, never a new installer fetch (R-840) | The builder (`scripts/build-config-bundle.py`) and the installer read the same table; `test_every_root_file_the_installer_writes_is_in_the_bundle` fails on a path the bundle lacks. Trust files (`/etc/felhom/os-trust.json`, `operator-signers`) are NEVER bundle paths (R17) |
 | `osupdate.ConfigUpdateExecutor` | internal/osupdate/bundle.go | signed op `agent_config_update` {agent_version, bundle_sha256} | delivering the bundle to an installed box | a courier only: the root wrapper re-verifies signature, host, nonce and sha itself |
 | `osupdate.Leg.SendUnsent` / `lockPass` (v0.144.0, R-868) | internal/osupdate/unsent.go | `(ctx) int` | an OS-pass report the agent never sent (killed mid-pass): the wrapper keeps `report-<run>-<layer>-apply.json` beside the plan; the agent deletes it once the hub has it | any new caller that runs an apply pass must hold `lockPass` (flock, across processes) — the sender must never take a running pass's copy |
 | `osupdate.LoadSavedBlock` (v0.144.0, R-866) | internal/osupdate/leg.go | `(planDir) (block, savedAt, ok)` | the hub's newest os_update block as the daemon last received it (`os-update-block.json`) | the debug pass uses it ONLY when the hub cannot be reached, and says so in its header; no saved block → no pass |
 | `dpkg_state()` / `DPKG_STATE_SCRIPT` (v0.145.0, R-876) | configs/felhom-os-apply | `audit, journal = self.dpkg_state()` | dpkg's state in ONE call: `--audit` AND the update journal | never gate a repair on `--audit` alone — a crash leaves only the journal (measured); keep it one call (R-845) |
+| `thincheck.Checker.Check` + `Describe` (v0.159.0, R-1012) | internal/thincheck/thincheck.go | `Check(ctx, vmid) []Finding` | name an ORPHAN thin device (pool holds it, LVM does not) in a whole-guest backup failure — `BackupRunner.SetThinCheck` | READ-ONLY by design: the root helper `felhom-thin-check <vg> <pool>` sends only reserve/release_metadata_snap; an orphan needs two reads; fails open (never stops a backup). The repair stays human |
 | `guesthook.SnippetReady` / `Register` (v0.146.0, R-861) | internal/guesthook/install.go | `SnippetReady(path) error` | is the pre-start hook (a FIXED file from the bundle) in place — register only then | The agent never installs the hook (Proxmox runs it as root); a missing hookscript stops a guest start, so never `Register` without `SnippetReady` |
 
 ### Disk / format safety (role gates, durable IDs, format guards)
@@ -56,6 +57,7 @@
 | `SudoHostOps.EnsureMount` | internal/storage/hostops.go | `EnsureMount(ctx, MountSpec) error` | persistent by-UUID systemd .mount | Validate→render→stage→`install`→`enable --now`; idempotent |
 | `SudoHostOps.Unmount` | internal/storage/hostops.go | `Unmount(ctx, where) error` | detach a mount unit | DESTRUCTIVE — caller MUST have gated it; does not self-authorize |
 | `SudoHostOps.ReassertEnrolledMounts` | internal/storage/hostops.go | `ReassertEnrolledMounts(ctx)` | reboot remount (re-resolve by UUID) | Re-asserts unless mounted AND enabled (`shouldReassertMount`); skips absent UUIDs |
+| `SudoHostOps.ForgetRetiredDrives` (v0.159.0, R-1124) | internal/storage/forget_retired.go | `ForgetRetiredDrives(ctx, IntentReader) int` | drop the host mount unit of a DECOMMISSIONED drive that is GONE (start-up + the 20 s tick) | Only decommissioned + absent + unmounted; a present decommissioned drive keeps its unit (H3 re-enroll). The file goes only through `felhom-priv-apply unit-forget`, which re-checks all three as root |
 | `GuestBinder.AttachDrive` / `DetachDrive` | internal/localapi/intermediary.go | `AttachDrive(ctx, vmid, where) (guestPath, err)` | live drive hot-swap under `/mnt/felhom-drives` | Normalizes to EXACTLY ONE bind via `countHostMounts` (converges double-binds); force re-bind when guest can't see it |
 | `GuestBinder.EnsureSharedParent` | internal/localapi/intermediary.go | `EnsureSharedParent(ctx) error` | shared-parent bind + boot unit | make-private+make-shared ONLY on first bind — re-running orphans the guest's slave; F2-a: compares script AND unit for staleness |
 | `StablePathForRaw` / `DriveNameFromRaw` | internal/localapi/intermediary.go | `StablePathForRaw("/mnt/<n>") string` | raw host mount → stable guest path | Single source of truth both repos derive from |
diff --git a/cmd/felhom-agent/main.go b/cmd/felhom-agent/main.go
index 6e84aee..7738d28 100644
--- a/cmd/felhom-agent/main.go
+++ b/cmd/felhom-agent/main.go
@@ -59,6 +59,7 @@ import (
 	"gitea.dooplex.hu/admin/felhom-agent/internal/selfupdate"
 	"gitea.dooplex.hu/admin/felhom-agent/internal/signedjobs"
 	"gitea.dooplex.hu/admin/felhom-agent/internal/storage"
+	"gitea.dooplex.hu/admin/felhom-agent/internal/thincheck"
 	"gitea.dooplex.hu/admin/felhom-agent/internal/vzpartial"
 	"gitea.dooplex.hu/admin/felhom-agent/internal/wgtunnel"
 )
@@ -1425,6 +1426,9 @@ func runDaemon(cfg config.Config, logger *slog.Logger, logRing *applog.Ring) int
 		// re-assert below sees its unit.
 		localSrv.EnsureInternalPlace(ctx)
 		if mountReasserter != nil {
+			// R-1124: a decommissioned drive that is gone loses its host mount unit first (start-up and every tick), so
+			// no later boot waits for it and the re-assert below no longer lists it.
+			mountReasserter.ForgetRetiredDrives(ctx, intentReader)
 			mountReasserter.ReassertEnrolledMounts(ctx)
 		}
 		// F9: on startup (the host's bring-up/reconcile trigger), re-assert any enrolled guest data-drive
@@ -1463,6 +1467,7 @@ func runDaemon(cfg config.Config, logger *slog.Logger, logRing *applog.Ring) int
 					// unit re-disabled at runtime), then re-assert the guest binds against the now-live mounts.
 					localSrv.EnsureInternalPlace(ctx) // `16` §5: a record written since the last tick (the signed job)
 					if mountReasserter != nil {
+						mountReasserter.ForgetRetiredDrives(ctx, intentReader) // R-1124: decommissioned + gone → unit removed
 						mountReasserter.ReassertEnrolledMounts(ctx)
 					}
 					localSrv.ReassertGuestBinds(ctx)
@@ -2061,8 +2066,12 @@ func buildLocalAPIServer(cfg config.Config, px *proxmox.Client, store *backup.St
 		},
 		Logger: logger,
 	}
+	// R-1012: an orphan thin device (a full host root during a snapshot) is NAMED in the backup failure the controller
+	// mails to the operator, through the read-only root helper. Never removed — the repair stays a human step.
+	thinChecker := &thincheck.Checker{API: px, Runner: &proxmox.ExecRunner{Mode: gaMode, SudoPath: cfg.Privileged.SudoPath}, Logger: logger}
 	for _, r := range tierRunners {
 		r.SetBeforeBackup(func(ctx context.Context, storage string, vmid int) { cleaner.Clean(ctx, storage, vmid) })
+		r.SetThinCheck(func(ctx context.Context, vmid int) string { return thincheck.Describe(thinChecker.Check(ctx, vmid)) })
 	}
 	srv.SetPartialSweep(func(ctx context.Context, vmid int) {
 		for _, t := range tierTargets {
diff --git a/configs/felhom-agent.sudoers b/configs/felhom-agent.sudoers
index 4b0c6ce..0f9d8a4 100644
--- a/configs/felhom-agent.sudoers
+++ b/configs/felhom-agent.sudoers
@@ -25,6 +25,7 @@
 
 Cmnd_Alias FELHOM_MOUNT = \
   /usr/local/sbin/felhom-priv-apply ^unit mnt-[A-Za-z0-9_.\\-]+\.(mount|automount)$, \
+  /usr/local/sbin/felhom-priv-apply ^unit-forget mnt-[A-Za-z0-9_.\\-]+\.mount$, \
   /usr/bin/systemctl daemon-reload, \
   /usr/bin/systemctl ^enable --now -- mnt-[A-Za-z0-9_.\\-]+\.mount$, \
   /usr/bin/systemctl ^disable -- mnt-[A-Za-z0-9_.\\-]+\.mount$, \
@@ -101,6 +102,11 @@ Cmnd_Alias FELHOM_VZPARTIAL = \
 # ruled size rule and formats ONLY that volume, ONLY when it carries no signature. The agent calls it from the signed
 # `add_internal_place` job (an old box), always with 0 GiB planned: its guest's volumes already exist. `plan` changes
 # nothing. The installer runs the helper as root itself (a new box).
+# R-1012 (agent v0.159.0): the READ-ONLY orphan-thin check before and after a whole-guest backup — lists the pool's thin
+# device ids (through a metadata snapshot it reserves and releases) next to LVM's. It sends no other pool message; the
+# repair stays a human step. Pinned by configs/test_felhom_thin_check.py.
+Cmnd_Alias FELHOM_THINCHECK = \
+  /usr/local/sbin/felhom-thin-check ^[A-Za-z0-9_+][A-Za-z0-9_.+-]{0\,63} [A-Za-z0-9_+][A-Za-z0-9_.+-]{0\,63}$
 Cmnd_Alias FELHOM_INTERNALPLACE = \
   /usr/local/sbin/felhom-internal-place ^(plan|create) 0$
 
@@ -336,4 +342,4 @@ Cmnd_Alias FELHOM_GUESTNET = \
   /usr/sbin/pct ^exec [0-9]+ -- pgrep -x dhclient$, \
   /usr/sbin/pct ^exec [0-9]+ -- dhclient -pf /run/dhclient\.eth0\.pid -lf /var/lib/dhcp/dhclient\.eth0\.leases eth0$
 
-felhom-agent ALL=(root) NOPASSWD: FELHOM_MOUNT, FELHOM_DISK, FELHOM_PROVISION, FELHOM_FORMAT, FELHOM_DNSMASQ, FELHOM_GUESTHOOK, FELHOM_INTERMEDIARY, FELHOM_CONTROLLERSWAP, FELHOM_STALELOCK, FELHOM_FSTRIM, FELHOM_NETMOUNT, FELHOM_WG, FELHOM_SELFUPDATE, FELHOM_SSHD, FELHOM_OOB, FELHOM_PBSDR, FELHOM_BACKUPTARGET, FELHOM_SELFHEAL, FELHOM_ESCROW, FELHOM_GUESTNET, FELHOM_SCRATCH_TEARDOWN, FELHOM_OSAPPLY, FELHOM_POWEROFF, FELHOM_VZPARTIAL, FELHOM_INTERNALPLACE
+felhom-agent ALL=(root) NOPASSWD: FELHOM_MOUNT, FELHOM_DISK, FELHOM_PROVISION, FELHOM_FORMAT, FELHOM_DNSMASQ, FELHOM_GUESTHOOK, FELHOM_INTERMEDIARY, FELHOM_CONTROLLERSWAP, FELHOM_STALELOCK, FELHOM_FSTRIM, FELHOM_NETMOUNT, FELHOM_WG, FELHOM_SELFUPDATE, FELHOM_SSHD, FELHOM_OOB, FELHOM_PBSDR, FELHOM_BACKUPTARGET, FELHOM_SELFHEAL, FELHOM_ESCROW, FELHOM_GUESTNET, FELHOM_SCRATCH_TEARDOWN, FELHOM_OSAPPLY, FELHOM_POWEROFF, FELHOM_VZPARTIAL, FELHOM_INTERNALPLACE, FELHOM_THINCHECK
diff --git a/configs/felhom-os-apply b/configs/felhom-os-apply
index 8fd8fe9..81565fb 100755
--- a/configs/felhom-os-apply
+++ b/configs/felhom-os-apply
@@ -191,6 +191,8 @@ BUNDLE_FILES = [
     ("/usr/local/sbin/felhom-backup-target-apply", "felhom-backup-target-apply", 0o755, "bash", "replace"),
     # R-1006 (agent v0.156.0): the only delete of backup files — a dead half-written vzdump, re-checked as root.
     ("/usr/local/sbin/felhom-vzdump-partial-clean", "felhom-vzdump-partial-clean", 0o755, "bash", "replace"),
+    # R-1012 (agent v0.159.0): the read-only orphan-thin check around a whole-guest backup (names an orphan, never removes).
+    ("/usr/local/sbin/felhom-thin-check", "felhom-thin-check", 0o755, "bash", "replace"),
     # felhom.eu architecture/16 §5 (agent v0.158.0): the internal data place — the only lvcreate/mkfs of a system-disk volume.
     ("/usr/local/sbin/felhom-internal-place", "felhom-internal-place", 0o755, "bash", "replace"),
     ("/usr/local/sbin/felhom-os-apply", "felhom-os-apply", 0o755, "python", "replace"),
diff --git a/configs/felhom-priv-apply b/configs/felhom-priv-apply
index 15c5d57..e21902e 100755
--- a/configs/felhom-priv-apply
+++ b/configs/felhom-priv-apply
@@ -13,6 +13,8 @@ longer name the destination: each verb has a fixed source and a fixed (or strict
 
 Verbs (each one sudoers line, exact-match pattern):
   unit <name>              /var/lib/felhom-agent/units/<name> -> /etc/systemd/system/<name>   (.mount | .automount)
+  unit-forget <name>       REMOVES /etc/systemd/system/<name> (R-1124): only a felhom by-UUID drive .mount unit that is
+                           disabled, not mounted, and whose drive is absent (its /dev/disk/by-uuid link is gone)
   dnsmasq <tmp> <name>     /tmp/felhom-resolver-<digits>.conf -> /etc/dnsmasq.d/felhom-<...>.conf
   wg                       /var/lib/felhom-agent/wg/wg-felhom.conf -> /etc/wireguard/wg-felhom.conf (0600)
   sshd-config              /var/lib/felhom-agent/felhom-sshd/sshd_config -> /etc/felhom-sshd/sshd_config
@@ -41,7 +43,7 @@ WG_SRC, WG_DEST = STATE + "/wg/wg-felhom.conf", "/etc/wireguard/wg-felhom.conf"
 SSHD_SRC, SSHD_DEST = STATE + "/felhom-sshd/sshd_config", "/etc/felhom-sshd/sshd_config"
 KEY_SRC, KEY_DEST = STATE + "/felhom-sshd/authorized_keys.felhom-op", "/etc/felhom-sshd/authorized_keys/felhom-op"
 MAX_BYTES = 64 * 1024
-VERBS = ("unit", "dnsmasq", "wg", "sshd-config", "sshd-key", "controller-image")
+VERBS = ("unit", "unit-forget", "dnsmasq", "wg", "sshd-config", "sshd-key", "controller-image")
 # R-861 (a) A1 (`09` §3 decision 165): the SAME pattern as the agent's controllerImageRe (internal/localapi/
 # controllerswap.go) — a compromised agent cannot hand the guest's bootstrap any other image. Pinned by
 # configs/test_felhom_priv_apply.py ControllerImage.
@@ -51,6 +53,11 @@ CONTROLLER_IMAGE_MAX = 256
 VMID_RE = re.compile(r"^[0-9]{1,9}$")
 
 UNIT_NAME_RE = re.compile(r"^mnt-[A-Za-z0-9_.\\-]+\.(mount|automount)$")
+DRIVE_UNIT_NAME_RE = re.compile(r"^mnt-[A-Za-z0-9_.\\-]+\.mount$")
+DRIVE_UNIT_MARKER = "Managed by felhom-agent"
+NET_UNIT_MARKER = "felhom network storage"
+WANTS_DIR = UNIT_DIR + "/multi-user.target.wants"
+BY_UUID_DIR = "/dev/disk/by-uuid"
 DNSMASQ_TMP_RE = re.compile(r"^/tmp/felhom-resolver-[0-9]+\.conf$")
 DNSMASQ_NAME_RE = re.compile(r"^felhom-[a-z0-9][a-z0-9._-]*\.conf$")
 SEG = r"[A-Za-z0-9_-][A-Za-z0-9_.-]*"
@@ -135,6 +142,36 @@ class Host:
     def read_stdin(self, limit):
         return sys.stdin.buffer.read(limit + 1)
 
+    # ---- unit-forget (R-1124) ----
+    def read_installed(self, path):
+        """The installed unit: None when absent; Refused when it is not a regular file (a symlink is never followed)."""
+        try:
+            fd = os.open(path, os.O_RDONLY | os.O_NOFOLLOW | os.O_CLOEXEC)
+        except FileNotFoundError:
+            return None
+        except OSError as e:
+            raise Refused("F1", f"cannot open {path}: {e.strerror}")
+        with os.fdopen(fd, "rb") as f:
+            if not stat.S_ISREG(os.fstat(f.fileno()).st_mode):
+                raise Refused("F1", f"{path} is not a regular file")
+            return f.read(MAX_BYTES + 1)
+
+    def lexists(self, path):
+        return os.path.lexists(path)
+
+    def mountpoints(self):
+        """Every mount point in /proc/self/mountinfo (field 5, octal escapes decoded)."""
+        out = set()
+        with open("/proc/self/mountinfo", "r", encoding="utf-8", errors="replace") as f:
+            for line in f:
+                parts = line.split(" ")
+                if len(parts) > 4:
+                    out.add(re.sub(r"\\([0-7]{3})", lambda m: chr(int(m.group(1), 8)), parts[4]))
+        return out
+
+    def remove(self, path):
+        os.remove(path)
+
     def write_guest_image(self, vmid, data):
         """As root: `pct exec <vmid> -- tee <the fixed file>` with the checked ref on stdin (no shell)."""
         r = subprocess.run(["/usr/sbin/pct", "exec", str(vmid), "--", "tee", CONTROLLER_IMAGE_FILE],
@@ -424,6 +461,45 @@ def controller_image(rest, host):
     return 0
 
 
+def unit_forget(rest, host):
+    """R-1124: remove ONE felhom drive mount unit that the agent has already disabled, whose drive is gone. A drive that
+    is still present or still mounted keeps its unit — this verb can only forget, never unmount or take a drive away."""
+    try:
+        if len(rest) != 1 or not DRIVE_UNIT_NAME_RE.match(rest[0]) or "/" in rest[0]:
+            raise Refused("A1", "usage: felhom-priv-apply unit-forget mnt-<escaped path>.mount")
+        name = rest[0]
+        path = os.path.join(UNIT_DIR, name)
+        data = host.read_installed(path)
+        if data is None:
+            host.log(f"felhom-priv-apply: ABSENT unit-forget {path} (nothing to remove)")
+            return 0
+        text = text_of(data, path)
+        if DRIVE_UNIT_MARKER not in text or NET_UNIT_MARKER in text:
+            raise Refused("F2", f"{name} is not a felhom drive unit")
+        check_unit(name, text)
+        s = parse_ini(text, name)
+        what, where = s["Mount"].get("What", ""), s["Mount"]["Where"]
+        mu = re.match(r"^/dev/disk/by-uuid/(.+)$", what)
+        if not mu:
+            raise Refused("F2", f"{name} is not a by-UUID drive unit")
+        if host.lexists(os.path.join(WANTS_DIR, name)):
+            raise Refused("F3", f"{name} is still enabled (disable it first)")
+        if where in host.mountpoints():
+            raise Refused("F4", f"{where} is mounted")
+        if host.lexists(os.path.join(BY_UUID_DIR, mu.group(1))):
+            raise Refused("F5", f"the drive of {name} is present (filesystem UUID {mu.group(1)})")
+    except Refused as e:
+        host.log(f"felhom-priv-apply: REFUSED [{e.rule}] unit-forget {' '.join(rest)[:120]}: {e.reason}")
+        return 2 if e.rule == "A1" else 3
+    try:
+        host.remove(path)
+    except OSError as e:
+        host.log(f"felhom-priv-apply: FAILED unit-forget {path}: {e}")
+        return 4
+    host.log(f"felhom-priv-apply: REMOVED unit-forget {path} (where={where})")
+    return 0
+
+
 def main(argv, host=None):
     host = host or Host()
     if argv == ["--self-check"]:
@@ -445,6 +521,8 @@ def main(argv, host=None):
         return 0
     if argv and argv[0] == "controller-image":
         return controller_image(argv[1:], host)
+    if argv and argv[0] == "unit-forget":
+        return unit_forget(argv[1:], host)
     try:
         verb, src, dest, mode, checker = plan(argv)
         data = host.read_source(src)
diff --git a/configs/felhom-thin-check b/configs/felhom-thin-check
new file mode 100755
index 0000000..35bb761
--- /dev/null
+++ b/configs/felhom-thin-check
@@ -0,0 +1,74 @@
+#!/bin/bash
+# felhom-thin-check — list the thin device ids a thin pool holds next to the ones LVM knows (R-1012). READ-ONLY: it
+# names an orphan, it never removes one. Installed as /usr/local/sbin/felhom-thin-check (config bundle).
+#
+#   felhom-thin-check <vg> <pool>
+#
+# A full host root while vzdump takes its snapshot can leave a thin snapshot inside the pool that LVM no longer knows
+# (demo-hp 2026-10-14: `thin_ls` device 4, LVM only 1 and 2); from then on every snapshot backup of that guest fails
+# („Creation of new snapshot 4 of device 2 failed") and nothing said why. The repair (`dmsetup message … delete <id>`)
+# stays a HUMAN step, decided by the operator — this helper only reads.
+#
+# Reading the live pool's device list needs a metadata snapshot: the kernel's documented way to read metadata while
+# the pool is in use (`reserve_metadata_snap`, `thin_ls --metadata-snap`, `release_metadata_snap`). It changes no data
+# and no mapping; it is held for the length of one `thin_ls -o DEV` (well under a second on these pools). Rules:
+#   1. the arguments are an LVM name pair; <vg>/<pool> is a thin pool (lv_attr starts with `t`);
+#   2. only one run at a time (a lock file); a metadata snapshot that someone ELSE holds is never touched — the run
+#      refuses (exit 3) and does not release it;
+#   3. LVM's list is read before AND after the pool's, so a volume created in between is not reported as an orphan
+#      (the caller treats an id as an orphan only when it is in the pool and in neither LVM list).
+# Output, one record per line:  lvm-before <id> <lv> | pool <id> | lvm-after <id> <lv> | ok
+# Exit: 0 read, 1 refused (arguments / not a thin pool), 3 a metadata snapshot is already held or cannot be reserved,
+# 4 the pool could not be read. Pinned by configs/test_felhom_thin_check.py.
+set -u
+LVS=/usr/sbin/lvs
+DMSETUP=/usr/sbin/dmsetup
+THIN_LS=/usr/sbin/thin_ls
+LOCK=/run/lock/felhom-thin-check.lock
+NAME_RE='^[A-Za-z0-9_+][A-Za-z0-9_.+-]{0,63}$'
+
+die() { echo "felhom-thin-check: REFUSED: $1" >&2; exit "${2:-1}"; }  # $1 the reason, $2 the exit code
+
+lvm_ids() { # $1 tag → "<tag> <id> <lv>" for every thin volume of this pool
+	"$LVS" --noheadings --separator , -o lv_name,thin_id --select "pool_lv=$POOL" -- "$VG" 2>/dev/null |
+		awk -F, -v tag="$1" '{ gsub(/ /, ""); if ($2 ~ /^[0-9]+$/) print tag, $2, $1 }'
+}
+
+release() { "$DMSETUP" message "$TPOOL" 0 release_metadata_snap >/dev/null 2>&1; }
+
+main() {
+	[[ $# -eq 2 ]] || die "usage: felhom-thin-check <vg> <pool>"
+	VG=$1 POOL=$2
+	local attr held ids
+	[[ "$VG" =~ $NAME_RE && "$POOL" =~ $NAME_RE ]] || die "not an LVM name pair"
+
+	attr=$("$LVS" --noheadings -o lv_attr -- "$VG/$POOL" 2>/dev/null | tr -d ' ')
+	[[ "$attr" == t* ]] || die "$VG/$POOL is not a thin pool"
+
+	DM="${VG//-/--}-${POOL//-/--}"
+	TPOOL="$DM-tpool"
+	TMETA="/dev/mapper/${DM}_tmeta"
+
+	exec 9>"$LOCK" || die "cannot open the lock file" 4
+	flock -w 30 9 || die "another check holds the lock" 3
+
+	# Field 7 of the thin-pool status is the held metadata root: "-" when no metadata snapshot exists.
+	held=$("$DMSETUP" status "$TPOOL" 2>/dev/null | awk '$3 == "thin-pool" { print $7 }')
+	[[ -n "$held" ]] || die "cannot read the status of $TPOOL" 4
+	[[ "$held" == "-" ]] || die "a metadata snapshot of $TPOOL is already held (by someone else) — not touched" 3
+
+	lvm_ids lvm-before
+	"$DMSETUP" message "$TPOOL" 0 reserve_metadata_snap >/dev/null 2>&1 || die "could not reserve a metadata snapshot of $TPOOL" 3
+	trap release EXIT
+	trap 'release; trap - EXIT; exit 4' INT TERM HUP
+	if ! ids=$("$THIN_LS" --metadata-snap --no-headers -o DEV "$TMETA" 2>/dev/null); then
+		die "thin_ls could not read $TMETA" 4
+	fi
+	release
+	trap - EXIT INT TERM HUP
+	awk '$1 ~ /^[0-9]+$/ { print "pool", $1 }' <<<"$ids"
+	lvm_ids lvm-after
+	echo ok
+}
+
+if [[ "${BASH_SOURCE[0]}" == "$0" ]]; then main "$@"; fi
diff --git a/configs/test_felhom_priv_apply.py b/configs/test_felhom_priv_apply.py
index c9bd97d..f210ac4 100644
--- a/configs/test_felhom_priv_apply.py
+++ b/configs/test_felhom_priv_apply.py
@@ -376,5 +376,67 @@ class ControllerImage(unittest.TestCase):
         self.assertIn("controller-image", buf.getvalue())
 
 
+class UnitForget(unittest.TestCase):
+    """R-1124: the agent forgets a decommissioned drive whose disk is gone — the root half removes its unit file, and
+    ONLY a felhom by-UUID drive unit that is disabled, not mounted, and whose /dev/disk/by-uuid link is absent.
+    RED-PROOF: on the pre-R-1124 wrapper `unit-forget` is not a verb (A1 usage, rc 2) — the removal case fails."""
+    NAME = "mnt-hdd_1.mount"
+    PATH = "/etc/systemd/system/mnt-hdd_1.mount"
+    UUID_LINK = "/dev/disk/by-uuid/91d2dc2d-2d28-4929-9bdd-3e11fa2f41ae"
+
+    def host(self, text=LOCAL_UNIT, kind="file", links=(), mounted=()):
+        h = FakeHost()
+        h.installed = {} if text is None else {self.PATH: (kind, text.encode())}
+        h.links, h.mounted, h.removed = set(links), set(mounted), []
+        h.read_installed = lambda p: (None if p not in h.installed else
+                                      (_ for _ in ()).throw(pa.Refused("F1", f"{p} is not a regular file"))
+                                      if h.installed[p][0] != "file" else h.installed[p][1])
+        h.lexists = lambda p: p in h.links
+        h.mountpoints = lambda: set(h.mounted)
+        h.remove = lambda p: h.removed.append(p)
+        return h
+
+    def test_an_absent_disabled_unmounted_drive_unit_is_removed(self):
+        h = self.host()
+        self.assertEqual(run(h, "unit-forget", self.NAME), 0, h.logs)
+        self.assertEqual(h.removed, [self.PATH])
+
+    def test_already_gone_is_fine(self):
+        h = self.host(text=None)
+        self.assertEqual(run(h, "unit-forget", self.NAME), 0, h.logs)
+        self.assertEqual(h.removed, [])
+
+    def refused(self, h, rule, *argv):
+        rc = run(h, *(argv or ("unit-forget", self.NAME)))
+        self.assertIn(rc, (2, 3), h.logs)
+        self.assertEqual(h.removed, [], h.logs)
+        self.assertTrue(any(f"[{rule}]" in l for l in h.logs), h.logs)
+
+    def test_F5_a_present_drive_keeps_its_unit(self):
+        self.refused(self.host(links=[self.UUID_LINK]), "F5")
+
+    def test_F4_a_mounted_drive_keeps_its_unit(self):
+        self.refused(self.host(mounted=["/mnt/hdd_1"]), "F4")
+
+    def test_F3_an_enabled_unit_is_not_removed(self):
+        self.refused(self.host(links=["/etc/systemd/system/multi-user.target.wants/" + self.NAME]), "F3")
+
+    def test_F2_a_network_unit_or_a_foreign_unit_is_not_removed(self):
+        self.refused(self.host(text=LOCAL_UNIT.replace("Managed by felhom-agent", "Written by someone")), "F2")
+        net = LOCAL_UNIT.replace("# Managed by felhom-agent", "# Managed by felhom-agent felhom network storage")
+        self.refused(self.host(text=net), "F2")
+
+    def test_F1_a_symlinked_unit_is_not_followed(self):
+        self.refused(self.host(kind="symlink"), "F1")
+
+    def test_A1_names(self):
+        for argv in (("unit-forget", "mnt-hdd_1.automount"), ("unit-forget", "sshd.service"),
+                     ("unit-forget", "../mnt-hdd_1.mount"), ("unit-forget",), ("unit-forget", self.NAME, "x")):
+            self.refused(self.host(), "A1", *argv)
+
+    def test_U3_a_unit_whose_name_lies_about_where_is_not_removed(self):
+        self.refused(self.host(text=LOCAL_UNIT.replace("Where=/mnt/hdd_1", "Where=/mnt/other")), "U3")
+
+
 if __name__ == "__main__":
     unittest.main(verbosity=2)
diff --git a/configs/test_felhom_thin_check.py b/configs/test_felhom_thin_check.py
new file mode 100644
index 0000000..e3e46f8
--- /dev/null
+++ b/configs/test_felhom_thin_check.py
@@ -0,0 +1,118 @@
+#!/usr/bin/env python3
+"""Tests for configs/felhom-thin-check (R-1012): it lists the thin pool's device ids next to LVM's and NEVER removes
+anything. Fake `lvs`, `dmsetup` and `thin_ls` record every call; nothing real is touched. Each refusal has a positive
+control (the healthy read), so a test cannot pass because the helper refuses everything.
+Run: python3 configs/test_felhom_thin_check.py   (also run by internal/thincheck's Go test)
+"""
+import os, pathlib, re, subprocess, tempfile, unittest
+
+HELPER = pathlib.Path(__file__).resolve().parent / "felhom-thin-check"
+
+
+class Box:
+    def __init__(self, pool_ids=(1, 2), lvm=(("vm-9201-disk-0", 1), ("vm-9201-disk-1", 2)), attr="twi-aotz--",
+                 held="-", reserve_rc=0, thin_ls_rc=0):
+        self.t = tempfile.TemporaryDirectory()
+        r = pathlib.Path(self.t.name)
+        self.log = r / "calls"
+        self.log.write_text("")
+        lvm_lines = "\\n".join(f"  {n},{i}" for n, i in lvm)
+        pool_lines = "\\n".join(str(i) for i in pool_ids)
+        self.bins = {}
+        for name, body in {
+            "lvs": f'''case "$*" in *lv_attr*) echo "  {attr}";; *thin_id*) printf "{lvm_lines}\\n";; esac''',
+            "dmsetup": f'''case "$1" in
+  status) echo "0 112975872 thin-pool 585 6860/262144 517080/882624 {held} rw discard_passdown queue_if_no_space - 1024";;
+  message) [ "$4" = reserve_metadata_snap ] && exit {reserve_rc}; exit 0;;
+esac''',
+            "thin_ls": f'''[ {thin_ls_rc} -ne 0 ] && exit {thin_ls_rc}; printf "{pool_lines}\\n"''',
+        }.items():
+            p = r / name
+            p.write_text(f'#!/bin/bash\necho "{name} $*" >> "{self.log}"\n{body}\n')
+            p.chmod(0o755)
+            self.bins[name] = p
+        self.lock = r / "lock"
+
+    def run(self, *args):
+        script = (f'source "{HELPER}"; LVS="{self.bins["lvs"]}"; DMSETUP="{self.bins["dmsetup"]}"; '
+                  f'THIN_LS="{self.bins["thin_ls"]}"; LOCK="{self.lock}"; main "$@"')
+        r = subprocess.run(["bash", "-c", script, "x", *args], capture_output=True, text=True)
+        return r.returncode, r.stdout, r.stderr
+
+    def calls(self):
+        return self.log.read_text().splitlines()
+
+
+def messages(calls):
+    return [c.split()[-1] for c in calls if c.startswith("dmsetup message")]
+
+
+class Reads(unittest.TestCase):
+    def test_healthy_pool_lists_both_sides_and_releases(self):
+        b = Box()
+        rc, out, err = b.run("pve", "data")
+        self.assertEqual(rc, 0, err)
+        self.assertEqual(out.splitlines(), ["lvm-before 1 vm-9201-disk-0", "lvm-before 2 vm-9201-disk-1",
+                                            "pool 1", "pool 2",
+                                            "lvm-after 1 vm-9201-disk-0", "lvm-after 2 vm-9201-disk-1", "ok"])
+        self.assertEqual(messages(b.calls()), ["reserve_metadata_snap", "release_metadata_snap"])
+        self.assertIn("thin_ls --metadata-snap --no-headers -o DEV /dev/mapper/pve-data_tmeta", b.calls())
+
+    def test_the_2026_10_14_orphan_is_listed(self):
+        rc, out, _ = Box(pool_ids=(1, 2, 4)).run("pve", "data")
+        self.assertEqual(rc, 0)
+        self.assertIn("pool 4", out.splitlines())
+
+    def test_dashed_names_map_to_the_device_mapper_name(self):
+        b = Box()
+        rc, _, err = b.run("felhom-scratch", "thin-pool")
+        self.assertEqual(rc, 0, err)
+        self.assertIn("dmsetup status felhom--scratch-thin--pool-tpool", b.calls())
+
+
+class Refuses(unittest.TestCase):
+    def refused(self, box, rc_want, *args):
+        rc, out, err = box.run(*args)
+        self.assertEqual(rc, rc_want, (out, err))
+        self.assertNotIn("ok", out.splitlines())
+        return box.calls()
+
+    def test_arguments(self):
+        for args in (("pve",), ("pve", "data", "x"), ("pve;id", "data"), ("-pve", "data"), ("pve", "../data"),
+                     ("pve", "da ta")):
+            calls = self.refused(Box(), 1, *args)
+            self.assertEqual(calls, [], args)
+
+    def test_not_a_thin_pool(self):
+        calls = self.refused(Box(attr="Vwi-aotz--"), 1, "pve", "vm-9201-disk-1")
+        self.assertEqual(messages(calls), [])
+
+    def test_a_snapshot_someone_else_holds_is_never_touched(self):
+        b = Box(held="1234")
+        rc, _, err = b.run("pve", "data")
+        self.assertEqual(rc, 3)
+        self.assertEqual(messages(b.calls()), [])  # neither reserve nor release
+        # the reason line ends with the reason, not the exit code (found live on demo-hp 2026-10-17)
+        self.assertTrue(err.strip().endswith("not touched"), err)
+
+    def test_reserve_failure_does_not_release(self):
+        calls = self.refused(Box(reserve_rc=1), 3, "pve", "data")
+        self.assertEqual(messages(calls), ["reserve_metadata_snap"])
+
+    def test_thin_ls_failure_still_releases(self):
+        calls = self.refused(Box(thin_ls_rc=1), 4, "pve", "data")
+        self.assertEqual(messages(calls), ["reserve_metadata_snap", "release_metadata_snap"])
+
+
+class NeverDeletes(unittest.TestCase):
+    def test_the_only_pool_messages_are_reserve_and_release(self):
+        src = HELPER.read_text()
+        sent = re.findall(r'"\$DMSETUP" message "\$TPOOL" 0 (\S+)', src)
+        self.assertEqual(sorted(set(sent)), ["release_metadata_snap", "reserve_metadata_snap"])
+        for word in ("delete", "lvremove", "thin_repair", "create_thin", "create_snap", " rm "):
+            code = "\n".join(l for l in src.splitlines() if not l.lstrip().startswith("#"))
+            self.assertNotIn(word, code, word)
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)
diff --git a/internal/backup/r1012_thincheck_test.go b/internal/backup/r1012_thincheck_test.go
new file mode 100644
index 0000000..571d6bb
--- /dev/null
+++ b/internal/backup/r1012_thincheck_test.go
@@ -0,0 +1,64 @@
+package backup
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-agent/internal/proxmox"
+)
+
+// R-1012: a snapshot backup that fails carries the orphan finding in its error — the text the controller mails to
+// the operator (`whole_guest_backup_failed`). The check runs once before the run and once, FRESH, after the failure.
+// RED-PROOF (P1/redproof-thincheck.txt): with explainFailure's append removed this test fails.
+func TestBackup_SnapshotFailure_NamesTheOrphan(t *testing.T) {
+	const why = "probable cause: thin pool pve/data holds device 4 that LVM does not know"
+	for name, api := range map[string]*fakeBackupAPI{
+		"task fails":  {vzdumpUPID: "UPID:x", waitErr: errors.New("task failed: job errors")},
+		"start fails": {vzdumpErr: errors.New("vzdump: 500")},
+	} {
+		t.Run(name, func(t *testing.T) {
+			r := NewBackupRunner(api, "local", "", "", "", quiet())
+			calls := 0
+			r.SetThinCheck(func(context.Context, int) string { calls++; return why })
+			rec, err := r.Backup(context.Background(), 9201)
+			if err == nil || rec.Success {
+				t.Fatalf("want a failure, got %+v", rec)
+			}
+			if !strings.Contains(rec.Error, why) {
+				t.Fatalf("the failure does not name the orphan: %q", rec.Error)
+			}
+			if calls != 2 {
+				t.Fatalf("thin check ran %d times, want 2 (before the run and after the failure)", calls)
+			}
+		})
+	}
+}
+
+// A successful backup keeps a clean record (the pre-check only logs); a stop-mode backup takes no snapshot, so the
+// check never runs; an unwired runner behaves exactly as before.
+func TestBackup_ThinCheck_SilentWhenNotApplicable(t *testing.T) {
+	ok := &fakeBackupAPI{vzdumpUPID: "UPID:x",
+		content: []proxmox.StorageContent{{VolID: "local:backup/vzdump-lxc-9201-x", Content: "backup", VMID: 9201, Size: 100, CTime: 1}}}
+	r := NewBackupRunner(ok, "local", "", "", "", quiet())
+	calls := 0
+	r.SetThinCheck(func(context.Context, int) string { calls++; return "probable cause: x" })
+	rec, err := r.Backup(context.Background(), 9201)
+	if err != nil || !rec.Success || rec.Error != "" || calls != 1 {
+		t.Fatalf("success: err=%v rec=%+v calls=%d (want one pre-check, clean record)", err, rec, calls)
+	}
+
+	stop := NewBackupRunner(&fakeBackupAPI{vzdumpUPID: "UPID:x", waitErr: errors.New("boom")}, "local", proxmox.ModeStop, "", "", quiet())
+	calls = 0
+	stop.SetThinCheck(func(context.Context, int) string { calls++; return "probable cause: x" })
+	rec, _ = stop.Backup(context.Background(), 9201)
+	if calls != 0 || strings.Contains(rec.Error, "probable cause") {
+		t.Fatalf("stop mode: calls=%d error=%q", calls, rec.Error)
+	}
+
+	plain := NewBackupRunner(&fakeBackupAPI{vzdumpUPID: "UPID:x", waitErr: errors.New("boom")}, "local", "", "", "", quiet())
+	if rec, _ := plain.Backup(context.Background(), 9201); rec.Error != "boom" {
+		t.Fatalf("unwired: error = %q, want the task error unchanged", rec.Error)
+	}
+}
diff --git a/internal/backup/runner.go b/internal/backup/runner.go
index 4726d13..9bc8f1e 100644
--- a/internal/backup/runner.go
+++ b/internal/backup/runner.go
@@ -61,8 +61,12 @@ type BackupRunner struct {
 	// beforeBackup runs just before the space preflight (R-1006): it removes a dead half-written vzdump of this guest on
 	// this target, so a leftover from a power cut cannot hold the space this backup needs. nil = nothing.
 	beforeBackup func(ctx context.Context, storage string, vmid int)
-	logger       *slog.Logger
-	now          func() time.Time
+	// thinCheck (R-1012) names an orphan thin device in the guest's pools in plain words ("" = none / not checked). It
+	// runs before a snapshot-mode backup (a WARN in the log) and again when the vzdump fails, so the failure the
+	// controller mails to the operator says why. It never removes anything. nil = not wired.
+	thinCheck func(ctx context.Context, vmid int) string
+	logger    *slog.Logger
+	now       func() time.Time
 	// rejected remembers the volids already announced by warnRejectedArchiveOnce, so an incomplete
 	// archive is reported ONCE rather than on every 5-minute due-check. Bounded in practice: one
 	// entry per aborted upload, and a process restart clears it. Guarded by rejectedMu because the
@@ -110,6 +114,22 @@ func (r *BackupRunner) SetBeforeBackup(fn func(ctx context.Context, storage stri
 	r.beforeBackup = fn
 }
 
+// SetThinCheck wires the R-1012 orphan check (see BackupRunner.thinCheck).
+func (r *BackupRunner) SetThinCheck(fn func(ctx context.Context, vmid int) string) { r.thinCheck = fn }
+
+// explainFailure appends the orphan finding to a failed snapshot-mode backup's error. A FRESH check (not the one from
+// before the run): the failure itself may be what made the orphan. Bounded, so a stuck helper cannot hold the job.
+func (r *BackupRunner) explainFailure(ctx context.Context, vmid int, rec *hub.Backup) {
+	if r.thinCheck == nil || r.mode != proxmox.ModeSnapshot {
+		return
+	}
+	cctx, cancel := context.WithTimeout(context.WithoutCancel(ctx), 2*time.Minute)
+	defer cancel()
+	if why := r.thinCheck(cctx, vmid); why != "" {
+		rec.Error = rec.Error + " — " + why
+	}
+}
+
 // localPruneSpec returns the `--prune-backups` spec to apply to THIS backup, or "" to skip pruning. It
 // applies the configured retention ONLY when the target is a non-PBS storage — PBS offsite retention is a
 // separate lifecycle and must never be pruned by the per-run flag (§9). Fail-safe: if the target's type
@@ -201,12 +221,18 @@ func (r *BackupRunner) backup(ctx context.Context, vmid int, onSnapshot func())
 		return rec, fmt.Errorf("backup: %s", rec.Error)
 	}
 
+	if r.thinCheck != nil && r.mode == proxmox.ModeSnapshot {
+		if why := r.thinCheck(ctx, vmid); why != "" {
+			r.logger.Warn("backup: an orphan thin device may make this snapshot backup fail (R-1012)", "vmid", vmid, "finding", why)
+		}
+	}
 	upid, err := r.api.Vzdump(ctx, proxmox.VzdumpOptions{
 		VMID: vmid, Storage: r.target, Mode: r.mode, Notes: r.notes,
 		PruneBackups: r.localPruneSpec(ctx), // local target → keep-last=N; PBS/unknown → "" (no prune)
 	})
 	if err != nil {
 		rec.Error = err.Error()
+		r.explainFailure(ctx, vmid, &rec)
 		rec.DurationSeconds = time.Since(start).Seconds()
 		return rec, fmt.Errorf("backup: vzdump vmid %d: %w", vmid, err)
 	}
@@ -220,6 +246,7 @@ func (r *BackupRunner) backup(ctx context.Context, vmid int, onSnapshot func())
 		}
 		if _, err := r.api.WaitTask(ctx, upid, proxmox.WaitOptions{Timeout: r.waitTimeout}); err != nil {
 			rec.Error = err.Error()
+			r.explainFailure(ctx, vmid, &rec)
 			rec.DurationSeconds = time.Since(start).Seconds()
 			return rec, fmt.Errorf("backup: vzdump task vmid %d: %w", vmid, err)
 		}
diff --git a/internal/capability/manifest.go b/internal/capability/manifest.go
index 2e843a5..3f1db5a 100644
--- a/internal/capability/manifest.go
+++ b/internal/capability/manifest.go
@@ -91,9 +91,11 @@ var manifest = []Capability{
 	{"disk-mkfs-xfs", "guarded format (xfs)", "/usr/local/sbin/felhom-mkfs-guarded", []string{"/dev/sda", "xfs"}, false, ""},
 	{"disk-smart", "disk SMART health read", "/usr/sbin/smartctl", []string{"-a", "-j", "/dev/sda"}, false, ""},
 	{"disk-lvs", "thin-pool usage read", "/usr/sbin/lvs", []string{"--reportformat", "json", "--units", "b", "-o", "lv_name,data_percent,metadata_percent", "--", "pve/data"}, false, ""},
+	{"thin-orphan-check", "orphan thin device check around a whole-guest backup (read-only, R-1012)", "/usr/local/sbin/felhom-thin-check", []string{"pve", "data"}, false, ""},
 
 	// ---- Storage mount units (watchdog re-mount) ----
 	{"mount-unit-install", "fs-UUID mount unit install (root content check, R-861)", "/usr/local/sbin/felhom-priv-apply", []string{"unit", "mnt-felhom\\x2dx.mount"}, false, ""},
+	{"mount-unit-forget", "decommissioned-drive unit removal (root check, R-1124)", "/usr/local/sbin/felhom-priv-apply", []string{"unit-forget", "mnt-felhom\\x2dx.mount"}, false, ""},
 	{"mount-daemon-reload", "systemd reload after unit write", "/usr/bin/systemctl", []string{"daemon-reload"}, false, ""},
 	{"mount-unit-enable", "mount unit enable", "/usr/bin/systemctl", []string{"enable", "--now", "--", "mnt-felhom\\x2dx.mount"}, false, ""},
 	{"mount-unit-disable", "mount unit disable", "/usr/bin/systemctl", []string{"disable", "--", "mnt-felhom\\x2dx.mount"}, false, ""},
diff --git a/internal/storage/forget_retired.go b/internal/storage/forget_retired.go
new file mode 100644
index 0000000..afda7f6
--- /dev/null
+++ b/internal/storage/forget_retired.go
@@ -0,0 +1,85 @@
+package storage
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// ForgetRetiredDrives (R-1124) removes the host mount unit of every DECOMMISSIONED drive that is physically gone, so a
+// later host boot no longer waits for it and the agent stops listing it (RegistryKnownTargets and
+// ReassertEnrolledMounts both enumerate the units — no unit, no entry, no „absent by UUID" line every 20 s).
+//
+// Only a drive that is BOTH decommissioned (the operator's or the customer's permanent removal) AND absent (its
+// /dev/disk/by-uuid link does not resolve) AND not mounted is forgotten. A decommissioned drive that is still plugged in
+// keeps its unit and its raw host mount — that is what makes the one-click re-enroll (H3) work, and decommission stays a
+// logical retire (`03` §storage). The intent record stays `decommissioned`: a forgotten drive that comes back is a new
+// candidate, never auto-mounted. The root half (`felhom-priv-apply unit-forget`) re-checks every one of these conditions
+// and refuses otherwise. Called at agent start, on the 20-second storage tick, and right after a decommission. Best
+// effort per unit; returns how many units it removed. Pinned by TestForgetRetiredDrives_*.
+func (h *SudoHostOps) ForgetRetiredDrives(ctx context.Context, intent IntentReader) int {
+	if intent == nil {
+		return 0 // no intent store → we cannot know a drive was retired; forget nothing
+	}
+	entries, err := os.ReadDir(h.unitDir)
+	if err != nil {
+		h.logger.Warn("storage: forget retired drives — cannot read unit dir", "dir", h.unitDir, "err", err)
+		return 0
+	}
+	mounted := map[string]bool{}
+	if ms, merr := h.host.Mounts(); merr == nil {
+		for _, m := range ms {
+			mounted[m.MountPoint] = true
+		}
+	} else {
+		h.logger.Warn("storage: forget retired drives — mount table unreadable; forgetting nothing", "err", merr)
+		return 0
+	}
+	removed := 0
+	for _, e := range entries {
+		name := e.Name()
+		if !strings.HasSuffix(name, ".mount") {
+			continue
+		}
+		data, rerr := os.ReadFile(filepath.Join(h.unitDir, name))
+		if rerr != nil {
+			continue
+		}
+		spec, ok := parseFelhomMountUnit(string(data))
+		if !ok {
+			continue // not one of our by-UUID drive units (network units are refused by the parser)
+		}
+		durableID := "uuid:" + spec.UUID
+		if intent.Get(durableID) != IntentDecommissioned {
+			continue
+		}
+		if mounted[spec.Where] {
+			h.logger.Debug("storage: decommissioned drive still mounted — keeping its unit", "where", spec.Where)
+			continue
+		}
+		if _, derr := ResolveStorageDevice(durableID); derr == nil {
+			h.logger.Debug("storage: decommissioned drive is present — keeping its unit (re-enroll stays one click)", "where", spec.Where)
+			continue
+		}
+		if h.unitEnabled(name) {
+			if err := h.run(ctx, h.bins.Systemctl, "disable", "--", name); err != nil {
+				h.logger.Warn("storage: forget retired drive — disable failed", "where", spec.Where, "unit", name, "err", err)
+				continue
+			}
+		}
+		if err := h.run(ctx, h.bins.PrivApply, "unit-forget", name); err != nil {
+			h.logger.Warn("storage: forget retired drive — unit removal refused or failed", "where", spec.Where, "unit", name, "err", err)
+			continue
+		}
+		removed++
+		h.logger.Info("storage: forgot a decommissioned drive that is gone — its mount unit is removed",
+			"name", spec.Name, "where", spec.Where, "uuid", spec.UUID, "unit", name)
+	}
+	if removed > 0 {
+		if err := h.run(ctx, h.bins.Systemctl, "daemon-reload"); err != nil {
+			h.logger.Warn("storage: daemon-reload after forgetting retired drives failed", "err", err)
+		}
+	}
+	return removed
+}
diff --git a/internal/storage/forget_retired_test.go b/internal/storage/forget_retired_test.go
new file mode 100644
index 0000000..ef33d65
--- /dev/null
+++ b/internal/storage/forget_retired_test.go
@@ -0,0 +1,155 @@
+package storage
+
+import (
+	"context"
+	"io"
+	"os"
+	"path/filepath"
+	"reflect"
+	"testing"
+)
+
+// forgetRunner records every privileged call and plays the root half of `felhom-priv-apply unit-forget`: the unit file
+// disappears from the unit dir, so the CONSEQUENCE (the drive is no longer listed) is observable, not just the calls.
+type forgetRunner struct {
+	unitDir string
+	calls   [][]string
+}
+
+func (r *forgetRunner) RunStdin(ctx context.Context, _ io.Reader, name string, args ...string) ([]byte, []byte, error) {
+	return r.Run(ctx, name, args...)
+}
+
+func (r *forgetRunner) Run(_ context.Context, name string, args ...string) ([]byte, []byte, error) {
+	r.calls = append(r.calls, append([]string{name}, args...))
+	if len(args) == 2 && args[0] == "unit-forget" {
+		_ = os.Remove(filepath.Join(r.unitDir, args[1]))
+	}
+	return nil, nil, nil
+}
+
+type forgetCase struct {
+	intent   DriveIntent
+	present  bool // the by-uuid link resolves
+	mounted  bool
+	enabled  bool // the wants-symlink exists
+	mountErr bool
+}
+
+const forgetUUID = "4ebfd36d-87d3-4b63-839e-a5a710fc799a"
+
+func runForget(t *testing.T, c forgetCase, intent IntentReader) (int, *forgetRunner, *RegistryKnownTargets, string) {
+	t.Helper()
+	unitDir := t.TempDir()
+	unit := "mnt-media_a.mount"
+	writeUnit(t, unitDir, unit, renderMountUnit(MountSpec{Name: forgetUUID, UUID: forgetUUID, Where: "/mnt/media_a", FSType: "ext4"}))
+	if c.enabled {
+		if err := os.MkdirAll(filepath.Join(unitDir, "multi-user.target.wants"), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.Symlink(filepath.Join(unitDir, unit), filepath.Join(unitDir, "multi-user.target.wants", unit)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	devRoot := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(devRoot, "by-uuid"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if c.present {
+		dev := filepath.Join(devRoot, "sdz1")
+		if err := os.WriteFile(dev, nil, 0o600); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.Symlink(dev, filepath.Join(devRoot, "by-uuid", forgetUUID)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	withDevDiskRoot(t, devRoot)
+	hr := &fakeHostReader{}
+	if c.mounted {
+		hr.mounts = []Mount{{MountPoint: "/mnt/media_a", FSType: "ext4"}}
+	}
+	if c.mountErr {
+		hr.mountsErr = os.ErrPermission
+	}
+	rr := &forgetRunner{unitDir: unitDir}
+	ops := NewSudoHostOps(SudoHostOpsConfig{Runner: rr, Bins: Binaries{}.withDefaults(), UnitDir: unitDir,
+		StageDir: t.TempDir(), Host: hr, Logger: quietLogger()})
+	if intent == nil {
+		intent = &fakeIntent{m: map[string]DriveIntent{"uuid:" + forgetUUID: c.intent}}
+	}
+	n := ops.ForgetRetiredDrives(context.Background(), intent)
+	return n, rr, NewRegistryKnownTargets(unitDir, intent, quietLogger()), unitDir
+}
+
+// The R-1124 case: decommissioned, then unplugged. The unit is disabled and removed through the root checker, systemd
+// is reloaded, and the CONSEQUENCE — the agent no longer lists the drive — holds. RED-PROOF (P1/redproof-forget.txt):
+// with ForgetRetiredDrives' body replaced by `return 0` this test fails (the drive is still listed).
+func TestForgetRetiredDrives_DecommissionedAndGone_UnitRemovedAndNotListed(t *testing.T) {
+	n, rr, known, _ := runForget(t, forgetCase{intent: IntentDecommissioned, enabled: true}, nil)
+	if n != 1 {
+		t.Fatalf("removed = %d, want 1 (calls %v)", n, rr.calls)
+	}
+	pa := Binaries{}.withDefaults()
+	want := [][]string{
+		{pa.Systemctl, "disable", "--", "mnt-media_a.mount"},
+		{pa.PrivApply, "unit-forget", "mnt-media_a.mount"},
+		{pa.Systemctl, "daemon-reload"},
+	}
+	if !reflect.DeepEqual(rr.calls, want) {
+		t.Fatalf("calls = %v\nwant    %v", rr.calls, want)
+	}
+	got, err := known.Known(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != 0 {
+		t.Fatalf("the forgotten drive is still listed: %+v", got)
+	}
+}
+
+// Already disabled (the Tester 1 shape after a manual disable): no disable call, the removal still runs.
+func TestForgetRetiredDrives_AlreadyDisabled_NoDisableCall(t *testing.T) {
+	n, rr, _, _ := runForget(t, forgetCase{intent: IntentDecommissioned}, nil)
+	pa := Binaries{}.withDefaults()
+	want := [][]string{{pa.PrivApply, "unit-forget", "mnt-media_a.mount"}, {pa.Systemctl, "daemon-reload"}}
+	if n != 1 || !reflect.DeepEqual(rr.calls, want) {
+		t.Fatalf("n=%d calls=%v want %v", n, rr.calls, want)
+	}
+}
+
+// Everything that must KEEP its unit: a decommissioned drive still plugged in (re-enroll is one click — H3), one still
+// mounted, an enrolled or ejected drive that is merely absent (it comes back), a drive with no recorded intent, and an
+// unreadable mount table. No privileged call at all, and the drive stays listed where it was listed.
+// RED-PROOF: with the present-drive check removed, the "present" case calls unit-forget and fails.
+func TestForgetRetiredDrives_KeepsEveryOtherUnit(t *testing.T) {
+	for name, c := range map[string]forgetCase{
+		"decommissioned but present": {intent: IntentDecommissioned, present: true, enabled: true},
+		"decommissioned but mounted": {intent: IntentDecommissioned, mounted: true},
+		"enrolled and absent":        {intent: IntentEnrolled},
+		"ejected and absent":         {intent: IntentEjected},
+		"no intent":                  {intent: IntentNew},
+		"mount table unreadable":     {intent: IntentDecommissioned, mountErr: true},
+	} {
+		t.Run(name, func(t *testing.T) {
+			n, rr, _, unitDir := runForget(t, c, nil)
+			if n != 0 || len(rr.calls) != 0 {
+				t.Fatalf("n=%d calls=%v — this unit must be kept", n, rr.calls)
+			}
+			if _, err := os.Stat(filepath.Join(unitDir, "mnt-media_a.mount")); err != nil {
+				t.Fatalf("unit file gone: %v", err)
+			}
+		})
+	}
+}
+
+// No intent store on the host (open failure): nothing is known to be retired, so nothing is forgotten.
+func TestForgetRetiredDrives_NilIntent_ForgetsNothing(t *testing.T) {
+	unitDir := t.TempDir()
+	writeUnit(t, unitDir, "mnt-media_a.mount", renderMountUnit(MountSpec{Name: "x", UUID: forgetUUID, Where: "/mnt/media_a"}))
+	rr := &forgetRunner{unitDir: unitDir}
+	ops := NewSudoHostOps(SudoHostOpsConfig{Runner: rr, UnitDir: unitDir, StageDir: t.TempDir(), Host: &fakeHostReader{}, Logger: quietLogger()})
+	if n := ops.ForgetRetiredDrives(context.Background(), nil); n != 0 || len(rr.calls) != 0 {
+		t.Fatalf("n=%d calls=%v", n, rr.calls)
+	}
+}
diff --git a/internal/thincheck/thincheck.go b/internal/thincheck/thincheck.go
new file mode 100644
index 0000000..6037268
--- /dev/null
+++ b/internal/thincheck/thincheck.go
@@ -0,0 +1,243 @@
+// Package thincheck finds an ORPHAN thin device: one the LVM thin pool holds that LVM itself no longer knows (R-1012).
+//
+// A full host root while vzdump takes its storage snapshot can leave such a device behind (demo-hp 2026-10-14: the
+// pool held devices 1, 2 and 4; LVM knew 1 and 2). From then on every snapshot backup of that guest fails at the
+// snapshot step („Creation of new snapshot 4 of device 2 failed") and nothing in the product said why. This package
+// NAMES the orphan, in plain words, in the backup's failure — it never removes one: the repair
+// (`dmsetup message <pool>-tpool 0 "delete <id>"`) stays a human step, decided by the operator (`03` whole-guest
+// backup; runbook `whole-guest-backup-orphan-thin.md`).
+//
+// The device list comes from the root helper `felhom-thin-check <vg> <pool>` (config bundle), which reads the pool
+// through a metadata snapshot and LVM's list before and after. An id is an orphan only when the pool holds it and
+// NEITHER LVM list names it, on two reads a few seconds apart — so a volume being created at that moment is never
+// reported.
+package thincheck
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+
+	"gitea.dooplex.hu/admin/felhom-agent/internal/proxmox"
+)
+
+// HelperPath is where the config bundle installs the root helper.
+const HelperPath = "/usr/local/sbin/felhom-thin-check"
+
+// Pool names one LVM thin pool.
+type Pool struct{ VG, Pool string }
+
+func (p Pool) String() string { return p.VG + "/" + p.Pool }
+
+// Reading is one helper run, parsed.
+type Reading struct {
+	PoolIDs   []int
+	LVMBefore map[int]string
+	LVMAfter  map[int]string
+}
+
+// Parse reads the helper's output. ok=false unless the closing "ok" line is there (a cut-off run is no reading).
+func Parse(out string) (Reading, bool) {
+	r := Reading{LVMBefore: map[int]string{}, LVMAfter: map[int]string{}}
+	done := false
+	for _, line := range strings.Split(out, "\n") {
+		f := strings.Fields(line)
+		switch {
+		case len(f) == 1 && f[0] == "ok":
+			done = true
+		case len(f) == 2 && f[0] == "pool":
+			if id, err := strconv.Atoi(f[1]); err == nil {
+				r.PoolIDs = append(r.PoolIDs, id)
+			}
+		case len(f) == 3 && (f[0] == "lvm-before" || f[0] == "lvm-after"):
+			id, err := strconv.Atoi(f[1])
+			if err != nil {
+				continue
+			}
+			if f[0] == "lvm-before" {
+				r.LVMBefore[id] = f[2]
+			} else {
+				r.LVMAfter[id] = f[2]
+			}
+		}
+	}
+	return r, done
+}
+
+// Orphans returns the ids the pool holds that neither LVM list names, sorted.
+func (r Reading) Orphans() []int {
+	var out []int
+	for _, id := range r.PoolIDs {
+		_, b := r.LVMBefore[id]
+		_, a := r.LVMAfter[id]
+		if !b && !a {
+			out = append(out, id)
+		}
+	}
+	sort.Ints(out)
+	return out
+}
+
+// Runner is the privileged exec surface (proxmox.ExecRunner in sudo mode).
+type Runner interface {
+	Run(ctx context.Context, name string, args ...string) ([]byte, []byte, error)
+}
+
+// StorageLister is the read the checker needs to map a guest's volumes to their thin pools (*proxmox.Client).
+type StorageLister interface {
+	GuestConfig(ctx context.Context, vmid int) (proxmox.GuestConfig, error)
+	ListStorage(ctx context.Context) ([]proxmox.Storage, error)
+}
+
+// Checker runs the check for one guest's pools.
+type Checker struct {
+	API    StorageLister
+	Runner Runner
+	Helper string        // "" → HelperPath
+	Pause  time.Duration // between the two reads when the first finds an orphan; 0 → 3 s
+	Logger *slog.Logger
+}
+
+// Finding is one pool with orphans.
+type Finding struct {
+	Pool    Pool
+	Orphans []int
+}
+
+// PoolsOf maps the guest's volume-backed disks (rootfs, mpN) to the lvmthin pools they live on.
+func (c *Checker) PoolsOf(ctx context.Context, vmid int) ([]Pool, error) {
+	cfg, err := c.API.GuestConfig(ctx, vmid)
+	if err != nil {
+		return nil, err
+	}
+	stores, err := c.API.ListStorage(ctx)
+	if err != nil {
+		return nil, err
+	}
+	thin := map[string]Pool{}
+	for _, s := range stores {
+		if s.Type == "lvmthin" && s.VGName != "" && s.ThinPool != "" {
+			thin[s.Storage] = Pool{VG: s.VGName, Pool: s.ThinPool}
+		}
+	}
+	vols := []string{cfg.RootFS}
+	mps := cfg.MountPoints()
+	keys := make([]string, 0, len(mps))
+	for k := range mps {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	for _, k := range keys {
+		vols = append(vols, mps[k])
+	}
+	seen := map[Pool]bool{}
+	var out []Pool
+	for _, v := range vols {
+		store, _, ok := strings.Cut(strings.Split(v, ",")[0], ":")
+		if !ok || strings.HasPrefix(store, "/") {
+			continue // a bind mount (a path) is no volume
+		}
+		if p, isThin := thin[store]; isThin && !seen[p] {
+			seen[p] = true
+			out = append(out, p)
+		}
+	}
+	return out, nil
+}
+
+// read runs the helper once.
+func (c *Checker) read(ctx context.Context, p Pool) (Reading, error) {
+	helper := c.Helper
+	if helper == "" {
+		helper = HelperPath
+	}
+	out, stderr, err := c.Runner.Run(ctx, helper, p.VG, p.Pool)
+	if err != nil {
+		return Reading{}, fmt.Errorf("felhom-thin-check %s: %w: %s", p, err, strings.TrimSpace(string(stderr)))
+	}
+	r, ok := Parse(string(out))
+	if !ok {
+		return Reading{}, fmt.Errorf("felhom-thin-check %s: the output has no closing ok line", p)
+	}
+	return r, nil
+}
+
+// Check returns the pools of vmid that hold an orphan (two reads, intersected). A pool that cannot be read is logged
+// and left out: the check FAILS OPEN — it explains a failure, it never stops a backup.
+func (c *Checker) Check(ctx context.Context, vmid int) []Finding {
+	logger := c.Logger
+	if logger == nil {
+		logger = slog.Default()
+	}
+	pools, err := c.PoolsOf(ctx, vmid)
+	if err != nil {
+		logger.Warn("thincheck: cannot map the guest's disks to thin pools — not checked", "vmid", vmid, "err", err)
+		return nil
+	}
+	pause := c.Pause
+	if pause <= 0 {
+		pause = 3 * time.Second
+	}
+	var found []Finding
+	for _, p := range pools {
+		first, err := c.read(ctx, p)
+		if err != nil {
+			logger.Warn("thincheck: thin pool not read — not checked", "vmid", vmid, "pool", p.String(), "err", err)
+			continue
+		}
+		o1 := first.Orphans()
+		if len(o1) == 0 {
+			logger.Debug("thincheck: thin pool and LVM agree", "vmid", vmid, "pool", p.String(), "devices", len(first.PoolIDs))
+			continue
+		}
+		select {
+		case <-ctx.Done():
+			return found
+		case <-time.After(pause):
+		}
+		second, err := c.read(ctx, p)
+		if err != nil {
+			logger.Warn("thincheck: second read failed — not reported", "vmid", vmid, "pool", p.String(), "err", err)
+			continue
+		}
+		in2 := map[int]bool{}
+		for _, id := range second.Orphans() {
+			in2[id] = true
+		}
+		var both []int
+		for _, id := range o1 {
+			if in2[id] {
+				both = append(both, id)
+			}
+		}
+		if len(both) > 0 {
+			found = append(found, Finding{Pool: p, Orphans: both})
+			logger.Warn("thincheck: ORPHAN thin device(s) in the pool — LVM does not know them; NOT removed (a human step)",
+				"vmid", vmid, "pool", p.String(), "orphans", both)
+		}
+	}
+	return found
+}
+
+// Describe is the operator sentence for a backup failure. Plain words; names the pool and the ids; says nothing was
+// removed and where the repair is written down.
+func Describe(f []Finding) string {
+	if len(f) == 0 {
+		return ""
+	}
+	var parts []string
+	for _, x := range f {
+		ids := make([]string, len(x.Orphans))
+		for i, id := range x.Orphans {
+			ids[i] = strconv.Itoa(id)
+		}
+		parts = append(parts, fmt.Sprintf("thin pool %s holds device %s that LVM does not know", x.Pool, strings.Join(ids, ", ")))
+	}
+	return "probable cause: " + strings.Join(parts, "; ") +
+		" (an orphan snapshot, usually left when the host disk filled during a backup). Snapshot backups on this pool " +
+		"fail until an operator removes it; Felhom did not remove anything. Repair: runbook whole-guest-backup-orphan-thin.md"
+}
diff --git a/internal/thincheck/thincheck_test.go b/internal/thincheck/thincheck_test.go
new file mode 100644
index 0000000..cedc6ff
--- /dev/null
+++ b/internal/thincheck/thincheck_test.go
@@ -0,0 +1,184 @@
+package thincheck
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"log/slog"
+	"os/exec"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-agent/internal/proxmox"
+)
+
+// The helper's output on demo-hp 2026-10-14, before the repair: the pool held 1, 2 and 4; LVM knew 1 and 2
+// (audits/power-cut-fixes-2026-10-14/E/thin-ls.txt).
+const orphanOut = `lvm-before 1 vm-9201-disk-0
+lvm-before 2 vm-9201-disk-1
+pool 1
+pool 2
+pool 4
+lvm-after 1 vm-9201-disk-0
+lvm-after 2 vm-9201-disk-1
+ok
+`
+
+const healthyOut = `lvm-before 1 vm-9201-disk-0
+lvm-before 2 vm-9201-disk-1
+pool 1
+pool 2
+lvm-after 1 vm-9201-disk-0
+lvm-after 2 vm-9201-disk-1
+ok
+`
+
+func quiet() *slog.Logger { return slog.New(slog.NewTextHandler(io.Discard, nil)) }
+
+func TestParse_TheOrphanOf20261014(t *testing.T) {
+	r, ok := Parse(orphanOut)
+	if !ok {
+		t.Fatal("a complete reading was not accepted")
+	}
+	if got := r.Orphans(); !reflect.DeepEqual(got, []int{4}) {
+		t.Fatalf("orphans = %v, want [4]", got)
+	}
+	h, _ := Parse(healthyOut)
+	if got := h.Orphans(); len(got) != 0 {
+		t.Fatalf("a healthy pool reported orphans %v", got)
+	}
+}
+
+// A volume LVM created WHILE the pool was read appears only in the after-list: never an orphan.
+func TestOrphans_AVolumeCreatedDuringTheReadIsNotAnOrphan(t *testing.T) {
+	r, _ := Parse(strings.Replace(orphanOut, "lvm-after 2 vm-9201-disk-1\n", "lvm-after 2 vm-9201-disk-1\nlvm-after 4 snap_vm-9201-disk-1_vzdump\n", 1))
+	if got := r.Orphans(); len(got) != 0 {
+		t.Fatalf("orphans = %v, want none", got)
+	}
+}
+
+// A run cut off before its closing "ok" (the helper died, the output is partial) is no reading at all.
+func TestParse_NoClosingLineIsNoReading(t *testing.T) {
+	if _, ok := Parse(strings.TrimSuffix(orphanOut, "ok\n")); ok {
+		t.Fatal("a cut-off run was accepted as a reading")
+	}
+}
+
+type fakeAPI struct {
+	cfg    proxmox.GuestConfig
+	stores []proxmox.Storage
+}
+
+func (f *fakeAPI) GuestConfig(context.Context, int) (proxmox.GuestConfig, error) { return f.cfg, nil }
+func (f *fakeAPI) ListStorage(context.Context) ([]proxmox.Storage, error)        { return f.stores, nil }
+
+func guest9201() *fakeAPI {
+	extra := map[string]json.RawMessage{
+		"mp0": json.RawMessage(`"local-lvm:vm-9201-disk-1,mp=/var/lib/felhom,backup=1,size=20G"`),
+		"mp8": json.RawMessage(`"/mnt/felhom-drives,mp=/mnt/felhom-drives"`),
+		"mp9": json.RawMessage(`"local:9201/vm-9201-disk-2.raw,mp=/x"`),
+	}
+	return &fakeAPI{
+		cfg: proxmox.GuestConfig{RootFS: "local-lvm:vm-9201-disk-0,size=8G", Extra: extra},
+		stores: []proxmox.Storage{
+			{Storage: "local", Type: "dir", Path: "/var/lib/vz"},
+			{Storage: "local-lvm", Type: "lvmthin", VGName: "pve", ThinPool: "data"},
+			{Storage: "felhom-pbs", Type: "pbs"},
+		},
+	}
+}
+
+type seqRunner struct {
+	outs  []string
+	errs  []error
+	calls [][]string
+}
+
+func (r *seqRunner) Run(_ context.Context, name string, args ...string) ([]byte, []byte, error) {
+	r.calls = append(r.calls, append([]string{name}, args...))
+	i := len(r.calls) - 1
+	var err error
+	if i < len(r.errs) {
+		err = r.errs[i]
+	}
+	if i < len(r.outs) {
+		return []byte(r.outs[i]), nil, err
+	}
+	return nil, nil, err
+}
+
+func TestPoolsOf_RootfsAndVolumeMountpointsOnly(t *testing.T) {
+	c := &Checker{API: guest9201(), Runner: &seqRunner{}, Logger: quiet()}
+	pools, err := c.PoolsOf(context.Background(), 9201)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(pools, []Pool{{VG: "pve", Pool: "data"}}) {
+		t.Fatalf("pools = %v, want [pve/data] once (bind mounts and dir volumes are not thin)", pools)
+	}
+}
+
+// The consequence: an orphan seen on BOTH reads becomes a finding whose sentence names the pool and the id, and says
+// nothing was removed. The only command ever run is the read-only helper with <vg> <pool>.
+// RED-PROOF (P1/redproof-thincheck.txt): with Orphans() returning nil this test fails.
+func TestCheck_OrphanOnBothReads_IsNamed(t *testing.T) {
+	rr := &seqRunner{outs: []string{orphanOut, orphanOut}}
+	c := &Checker{API: guest9201(), Runner: rr, Pause: 1, Logger: quiet()}
+	f := c.Check(context.Background(), 9201)
+	if len(f) != 1 || f[0].Pool.String() != "pve/data" || !reflect.DeepEqual(f[0].Orphans, []int{4}) {
+		t.Fatalf("findings = %+v", f)
+	}
+	want := [][]string{{HelperPath, "pve", "data"}, {HelperPath, "pve", "data"}}
+	if !reflect.DeepEqual(rr.calls, want) {
+		t.Fatalf("calls = %v, want only the read-only helper twice", rr.calls)
+	}
+	d := Describe(f)
+	for _, s := range []string{"thin pool pve/data holds device 4 that LVM does not know", "Felhom did not remove anything",
+		"whole-guest-backup-orphan-thin.md"} {
+		if !strings.Contains(d, s) {
+			t.Fatalf("Describe = %q, missing %q", d, s)
+		}
+	}
+}
+
+// An id that is gone on the second read (a volume mid-creation) is not reported; a healthy pool is read ONCE.
+func TestCheck_TransientOrHealthy_NothingReported(t *testing.T) {
+	rr := &seqRunner{outs: []string{orphanOut, healthyOut}}
+	if f := (&Checker{API: guest9201(), Runner: rr, Pause: 1, Logger: quiet()}).Check(context.Background(), 9201); len(f) != 0 {
+		t.Fatalf("a transient id was reported: %+v", f)
+	}
+	rr = &seqRunner{outs: []string{healthyOut}}
+	if f := (&Checker{API: guest9201(), Runner: rr, Logger: quiet()}).Check(context.Background(), 9201); len(f) != 0 || len(rr.calls) != 1 {
+		t.Fatalf("healthy: findings %+v, calls %v", f, rr.calls)
+	}
+	if Describe(nil) != "" {
+		t.Fatal("no finding must describe as empty")
+	}
+}
+
+// The helper refusing (a metadata snapshot held by someone else, exit 3) or failing: no finding, no panic — the check
+// fails open; it explains a failure, it never stops a backup.
+func TestCheck_HelperRefuses_FailsOpen(t *testing.T) {
+	rr := &seqRunner{errs: []error{errors.New("exit status 3")}}
+	if f := (&Checker{API: guest9201(), Runner: rr, Logger: quiet()}).Check(context.Background(), 9201); len(f) != 0 {
+		t.Fatalf("findings = %+v", f)
+	}
+}
+
+// The root helper's own tests (configs/test_felhom_thin_check.py) run with the Go suite.
+func TestRootHelperPythonSuite(t *testing.T) {
+	py, err := exec.LookPath("python3")
+	if err != nil {
+		t.Skip("python3 not found")
+	}
+	_, file, _, _ := runtime.Caller(0)
+	script := filepath.Join(filepath.Dir(file), "..", "..", "configs", "test_felhom_thin_check.py")
+	out, err := exec.Command(py, script).CombinedOutput()
+	if err != nil {
+		t.Fatalf("helper suite failed: %v\n%s", err, out)
+	}
+}