The audit said the union row "writes false" for the guest-path key. That is
wrong, and the next reader would have inherited the error.
Isolated during R-116's Phase 0: RoleForStorage returns RoleSystem whenever
backingDevice == "" (felhom-agent internal/storage/role.go:180-181). When the
device vanishes the target row's role flips to system and it loses its guest
path, but KEEPS its MountPath -- and the union loop skips any drive whose
MountPath is already seen, so the registry row is never emitted at all.
/disks therefore carries NO row with that guest path: isTarget[guestPath] is a
MISSING KEY, not a false value. The practical difference is decisive -- the
obvious fix (set BackupTarget on the union row) could not have worked, because
that row does not exist in the state where the alarm is needed.
The section's own "not isolated" caveat is replaced by the isolated answer.
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