v0.317.0: a second copy counts only on a proven different disk; the no-drive install speaks in words (felhom.eu architecture/16, build A)

Tier-2 and the one-copy notice ask the host agent (POST /disks/same-disk, agent 0.157.0): different +
proven -> made and counted; same disk -> not made (D1); unknown (VM, hidden USB serial) -> made, not
counted, the row says why (D2); a NAS never counts and no longer hides the notice (R-1075). An older
agent keeps the st_dev rule, logged once. D4: a data-folder or media app on a box with no drive is
refused in words with a link to the drives page (R-1068's raw-error half). MinAgent 0.157.0.

Claude-Session: https://claude.ai/code/session_012qRErfCoiTkvDK9N5XHbzb

REUSE.md

@@ -214,6 +214,7 @@
 | `registerStoragePath` | controller/internal/web/storage_handlers.go | `(where, label, setDefault) error` | Post-enroll registration | The single funnel used by init/attach/manual-add |
 | `system.IsMountPoint` / `IsWritable` / `PathsOverlap` | controller/internal/system/mounts_linux.go | `(path) bool` | Mount checks | `_other.go` stubs return permissive values — Linux behavior is the real one |
 | `system.CheckBackupDestination` | controller/internal/system/mounts_linux.go | `(path) DestinationHealth` | Tier2/offbox target vetting | Detects same-physical-device (`SamePhysicalDevice`) |
+| `backup.Manager.diskRelation` | controller/internal/backup/backup.go | `(a, b) (DiskRelation, reason)` — DiskDifferent / DiskSame / DiskUnknown | EVERY Tier-2 or one-copy "same disk?" decision (v0.317.0, felhom.eu `16-storage-physical-identity.md` §4) | Asks the host agent (`SetDiskJudge`, FeatureSameDisk ≥ agent 0.157.0); an older agent or an `unmapped` path → the old `sameDevice`/st_dev rule. Never call `sameDevice` directly for a Tier-2 decision — st_dev calls two partitions of one disk "different". Pinned by diskrelation_test.go |
 | `system.ProbeStoragePath` | controller/internal/system/mounts_linux.go | `(path) ProbeResult` | Disconnect detection | — |
 | `appexport.DiskFree` | controller/internal/appexport/estimate.go | `(path) int64` | Free bytes for space gates (df-based, 0 on any error) | Exported v0.128.0 for the browser-upload gate; test seam = `web.uploadDiskFree` package var |
 | `stacks.ExportDataMounts` | controller/internal/stacks/delete.go | `(composePath, hddPath) []string` | THE .fab-export mount discovery (v0.130.0 C6B-F1) | Unions `${HDD_PATH}` binds + the `${USERDATA_PATH}` ROOT (single `userdata` entry — basename must round-trip the import's `<HDD_PATH>/<subdir>` mapping; NEVER return per-bind userdata subpaths). Containment-deduped. Backup-side `stackAdapter` deliberately does NOT use it |