TrueNAS SCALE Pool Will Not Mount After Drive Replacement
When a TrueNAS SCALE pool stops mounting after a drive replacement, the visible replacement event may hide a second problem: incomplete resilvering, a member that left the vdev, conflicting ZFS labels, device-path changes, or unreadable regions on a surviving disk.
Do not create a replacement pool, clear labels, detach another member, or use forced import as a repeated test. Keep the original and replacement drives identified and preserve the current topology.
Tell an ADR technician what the system reports and what has already been attempted. The failed system does not have to be brought online first.
What this condition means
SCALE may detect every disk while ZFS still refuses the pool because the member GUIDs, vdev roles, or transaction history do not describe a state it can safely import.
The replacement drive can contain a partial resilver rather than a complete copy. The original member may also retain readable regions needed to reconstruct the last consistent pool.
What to record
- TrueNAS SCALE release and recent updates
- Exact pool status, alert, and import output
- Pool name, vdev layout, RAIDZ level, and member count
- Bay, serial number, and device path for every original and replacement disk
- Replacement, resilver percentage, reboot, offline, detach, or import history
What not to do
- Create a new pool over the affected disks
- Clear ZFS labels or wipe an offered disk
- Repeat forced imports without preserving evidence
- Detach or replace another member by guesswork
- Discard the original member or partial resilver target
How ADR evaluates it
ADR establishes physical readability and compares the ZFS labels, vdev identities, transaction groups, and replacement history across all available members.
Logical cases with stable disks may be handled remotely. Unstable sources are imaged first, after which candidate pool states and priority datasets are validated away from the original NAS.
The original member and the partial resilver target may each contain useful evidence. ADR can compare both outside the active pool and determine the safest reconstruction path without requiring SCALE to import or repair the affected pool first.
What happens next
Many post-replacement SCALE failures retain recoverable datasets. The result turns on vdev redundancy, member readability, resilver progress, and any writes made after the pool stopped mounting.