diff --git a/CHANGELOG.md b/CHANGELOG.md
index 782de66..f9fb0a3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+## Unreleased (2026-10-06 night) — the burn-down night's controller fixes; ships with the next release
+
+**MinAgent: 0.131.0** (unchanged — no new agent route is read).
+
+- **R-542:** `GET /api/disks/candidates` no longer offers a drive that backs a REGISTERED storage path — not under `initialize` (the format wizard renders that list as-is, so a household's in-use data drive was offered for formatting) and not again under `attach`. The join is the guest's own mount table (a registered path's mount source is the host device). Fail-safe: an unreadable mount table empties `initialize`; `attach` passes through. Tests `TestR542_*` (two red-proofs observed).
+
 ## v0.301.0 — one short stop per backup tier; restores replay before the app starts; the family window reopens a command-closed sign-up (R-518, R-638, R-717; `09` §3 154, 156) (2026-10-06)
 
 **MinAgent: 0.131.0** (unchanged — R-518 reads the agent's existing `Primary` tier flag, R-82 agents ≥ 0.97.0).
diff --git a/controller/internal/web/agent_disk_handlers.go b/controller/internal/web/agent_disk_handlers.go
index e031f45..6a1792c 100644
--- a/controller/internal/web/agent_disk_handlers.go
+++ b/controller/internal/web/agent_disk_handlers.go
@@ -5,7 +5,9 @@ import (
 	"encoding/json"
 	"errors"
 	"net/http"
+	"path"
 	"sort"
+	"strings"
 
 	"gitea.dooplex.hu/admin/felhom-controller/internal/agentapi"
 )
@@ -126,9 +128,10 @@ func (s *Server) agentDisksListHandler(w http.ResponseWriter, r *http.Request) {
 
 // agentDiskCandidatesHandler proxies GET /api/disks/candidates → agent GET /disks/candidates (Impl-2b):
 // the raw-device scan (Impl-2a) that feeds the enrollment wizards. The agent's unclaimed-disk filter
-// already excludes claimed/OS/enrolled disks (fail-safe), so `initialize` passes through UNTOUCHED —
-// no controller-side filtering, and the system/backup drives it hides from the format wizard stay
-// hidden.
+// already excludes claimed/OS disks (fail-safe), and the system/backup drives it hides from the format
+// wizard stay hidden. It deliberately does NOT exclude Felhom's own drives, so the controller removes
+// every drive backing a REGISTERED storage path from both lists (R-542, dropRegisteredDrives) — the
+// one filter the controller adds; it only ever removes entries.
 //
 // R-280: `attach` additionally carries the controller's own mounted-but-unregistered filesystems.
 // The agent's scan alone left a rebuilt box with an empty picker under a sentence promising „két
@@ -150,9 +153,76 @@ func (s *Server) agentDiskCandidatesHandler(w http.ResponseWriter, r *http.Reque
 		writeDiskJSON(w, http.StatusBadGateway, false, s.errText(r, err), nil)
 		return
 	}
+	mounts := readMountTable()
+	resp = dropRegisteredDrives(resp, mounts, s.registeredStoragePaths())
 	writeDiskJSON(w, http.StatusOK, true, "", mergeAttachCandidates(resp, s.attachableStores()))
 }
 
+// dropRegisteredDrives (R-542) removes from BOTH lists every agent candidate that backs a REGISTERED
+// storage path. The agent deliberately lets Felhom re-initialise its own drives (a mount under
+// /mnt/felhom-drives is not a foreign claim — felhom-agent internal/storage/claim.go), so a drive the
+// household registered and uses came back under `initialize` — the format wizard (storage_init.html)
+// renders that list as-is. Measured 2026-09-16 on a fresh box: the registered default data drive was
+// offered for formatting and again under `attach`.
+//
+// The join is the guest's own mount table: a registered path's mount SOURCE is the host device
+// (measured on demo-hp 2026-10-06: `/dev/nvme0n1 /mnt/felhom-drives/hdd_1`), which is the candidate's
+// Device (whole disk) or MountSource (partition).
+//
+// ⚠ FAIL-SAFE: an unreadable mount table cannot prove any candidate is NOT in use, so `initialize`
+// comes back EMPTY (the wizard then says no drive is available) — never the unfiltered list. `attach`
+// is non-destructive and is left as the agent sent it in that case.
+func dropRegisteredDrives(resp agentapi.CandidatesResult, mountsText string, registered map[string]bool) agentapi.CandidatesResult {
+	if strings.TrimSpace(mountsText) == "" {
+		resp.Initialize = []agentapi.DiskCandidate{}
+		return resp
+	}
+	inUse := map[string]bool{}
+	for _, row := range parseMountTable(mountsText) {
+		if registered[path.Clean(row[1])] && strings.HasPrefix(row[0], "/dev/") {
+			inUse[row[0]] = true
+		}
+	}
+	if len(inUse) == 0 {
+		return resp
+	}
+	keep := func(in []agentapi.DiskCandidate) []agentapi.DiskCandidate {
+		out := make([]agentapi.DiskCandidate, 0, len(in))
+		for _, c := range in {
+			if candidateBacksInUse(c, inUse) {
+				continue
+			}
+			out = append(out, c)
+		}
+		return out
+	}
+	resp.Initialize = keep(resp.Initialize)
+	resp.Attach = keep(resp.Attach)
+	return resp
+}
+
+// candidateBacksInUse reports whether a candidate's disk is a source in inUse: the whole disk itself,
+// its mountable partition, or any partition of it (/dev/sdb1, /dev/nvme0n1p1 under /dev/sdb, /dev/nvme0n1).
+func candidateBacksInUse(c agentapi.DiskCandidate, inUse map[string]bool) bool {
+	if inUse[c.Device] || (c.MountSource != "" && inUse[c.MountSource]) {
+		return true
+	}
+	if c.Device == "" {
+		return false
+	}
+	for src := range inUse {
+		rest, ok := strings.CutPrefix(src, c.Device)
+		if !ok || rest == "" {
+			continue
+		}
+		rest = strings.TrimPrefix(rest, "p")
+		if rest != "" && strings.Trim(rest, "0123456789") == "" {
+			return true
+		}
+	}
+	return false
+}
+
 // mergeAttachCandidates adds the mounted-but-unregistered stores to `attach` and returns the result.
 // `initialize` is passed through untouched — the ONE line that keeps the format wizard's protection
 // intact, and the reason this is a separate function rather than two appends at the call site: it can
diff --git a/controller/internal/web/r542_registered_drive_test.go b/controller/internal/web/r542_registered_drive_test.go
new file mode 100644
index 0000000..9459370
--- /dev/null
+++ b/controller/internal/web/r542_registered_drive_test.go
@@ -0,0 +1,77 @@
+package web
+
+import (
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-controller/internal/agentapi"
+)
+
+// R-542 — a drive that backs a REGISTERED storage path must not be offered for formatting (nor again
+// for attaching). The mount table is the shape measured on demo-hp 2026-10-06 (guest /proc/mounts).
+const r542Mounts = `/dev/mapper/pve-root /mnt/felhom-drives ext4 rw,relatime 0 0
+/dev/nvme0n1 /mnt/felhom-drives/hdd_1 ext4 rw,relatime 0 0
+/dev/sdb1 /mnt/felhom-drives/adatlemez ext4 rw,relatime 0 0
+/dev/mapper/pve-vm--9201--disk--1 /mnt/sys_drive ext4 rw,relatime 0 0
+`
+
+// COMPANION RED-PROOF (observed): make dropRegisteredDrives `return resp` at its top (the pre-R-542
+// pass-through) → this fails with "a registered, in-use drive is offered for FORMATTING". Restored.
+func TestR542_RegisteredDriveNotOfferedForFormat(t *testing.T) {
+	agentSaid := agentapi.CandidatesResult{
+		Initialize: []agentapi.DiskCandidate{
+			{Device: "/dev/sdb", MountSource: "/dev/sdb1", FSType: "ext4", DataBearing: true}, // registered (partition source)
+			{Device: "/dev/nvme0n1", FSType: "ext4", DataBearing: true},                       // registered (whole-disk source)
+			{Device: "/dev/sdc"}, // a fresh, blank drive
+		},
+		Attach: []agentapi.DiskCandidate{
+			{Device: "/dev/sdb", MountSource: "/dev/sdb1", FSType: "ext4"},
+			{Device: "/dev/sdd", MountSource: "/dev/sdd1", FSType: "ext4"}, // a fresh USB with a filesystem
+		},
+	}
+	registered := map[string]bool{"/mnt/felhom-drives/hdd_1": true, "/mnt/felhom-drives/adatlemez": true, "/mnt/sys_drive": true}
+	got := dropRegisteredDrives(agentSaid, r542Mounts, registered)
+	if len(got.Initialize) != 1 || got.Initialize[0].Device != "/dev/sdc" {
+		t.Fatalf("a registered, in-use drive is offered for FORMATTING: initialize=%+v (want only /dev/sdc)", got.Initialize)
+	}
+	if len(got.Attach) != 1 || got.Attach[0].Device != "/dev/sdd" {
+		t.Fatalf("a registered drive is offered again under attach: %+v (want only /dev/sdd)", got.Attach)
+	}
+}
+
+// An UNregistered drive mounted under the managed path keeps today's behaviour (the agent's rule:
+// re-initialising Felhom's own, unregistered drive stays allowed — e.g. after a rebuild).
+func TestR542_UnregisteredDriveStillOffered(t *testing.T) {
+	agentSaid := agentapi.CandidatesResult{Initialize: []agentapi.DiskCandidate{{Device: "/dev/sdb", MountSource: "/dev/sdb1"}}}
+	got := dropRegisteredDrives(agentSaid, r542Mounts, map[string]bool{"/mnt/sys_drive": true})
+	if len(got.Initialize) != 1 {
+		t.Fatalf("an unregistered drive must stay offered; got %+v", got.Initialize)
+	}
+}
+
+// FAIL-SAFE: an unreadable mount table cannot prove a drive is free → initialize is EMPTY, attach unchanged.
+//
+// COMPANION RED-PROOF (observed): drop the empty-mounts branch → this fails with "unreadable mount
+// table must empty initialize". Restored.
+func TestR542_UnreadableMountTableEmptiesInitialize(t *testing.T) {
+	agentSaid := agentapi.CandidatesResult{
+		Initialize: []agentapi.DiskCandidate{{Device: "/dev/sdb"}},
+		Attach:     []agentapi.DiskCandidate{{Device: "/dev/sdd", MountSource: "/dev/sdd1"}},
+	}
+	got := dropRegisteredDrives(agentSaid, "", map[string]bool{"/mnt/felhom-drives/adatlemez": true})
+	if len(got.Initialize) != 0 || got.Initialize == nil {
+		t.Fatalf("unreadable mount table must empty initialize (non-nil, so JSON is []); got %+v", got.Initialize)
+	}
+	if len(got.Attach) != 1 {
+		t.Fatalf("attach is non-destructive and must pass through; got %+v", got.Attach)
+	}
+}
+
+// Partition matching must not over-match: /dev/sdb1 in use does not hide /dev/sdba or /dev/sdc.
+func TestR542_PartitionMatchIsExact(t *testing.T) {
+	inUse := map[string]bool{"/dev/sdb1": true, "/dev/nvme0n1p2": true}
+	for dev, want := range map[string]bool{"/dev/sdb": true, "/dev/sdba": false, "/dev/sdc": false, "/dev/nvme0n1": true, "/dev/nvme0n12": false} {
+		if got := candidateBacksInUse(agentapi.DiskCandidate{Device: dev}, inUse); got != want {
+			t.Errorf("candidateBacksInUse(%s) = %v, want %v", dev, got, want)
+		}
+	}
+}
diff --git a/controller/internal/web/templates/storage_init.html b/controller/internal/web/templates/storage_init.html
index 7ed0a2b..2383322 100644
--- a/controller/internal/web/templates/storage_init.html
+++ b/controller/internal/web/templates/storage_init.html
@@ -75,8 +75,8 @@ function fmtSize(b){
 async function loadDisks(){
   try{
     // Impl-2b: the raw-device scan (agent GET /disks/candidates via the controller proxy). The agent's
-    // unclaimed-disk filter already excludes OS/enrolled/claimed disks (fail-safe), so we trust the list
-    // as-is — no client-side filtering. `initialize` = every unclaimed disk (blank or data-bearing).
+    // unclaimed-disk filter excludes OS/claimed disks (fail-safe) and the proxy removes every drive behind a
+    // registered storage path (R-542), so we trust the list as-is — no client-side filtering. `initialize` = every unclaimed disk (blank or data-bearing).
     var r = await fetch('/api/disks/candidates'); var j = await r.json();
     if(!j.ok){ throw new Error(j.error||'{{T "storage_init.hiba"}}'); }
     var cands = (j.data&&j.data.initialize)||[];