Why did my RAID rebuild fail?

A failed rebuild is stressful because it often means data loss, but the causes are a short list.

The most common cause: a second drive faltered

A rebuild reads every surviving drive end to end to reconstruct the missing one. That full-surface read under sustained load is exactly when a marginal second drive throws errors or drops out. On a single-parity array (RAID 5 / SHR) that has no remaining redundancy during the rebuild, a second drive failing means the pool crashes and the rebuild fails. This is the classic RAID 5 failure story, and the reason large arrays prefer RAID 6 / SHR-2.

Other causes

• SMR replacement drive: its write speed collapses under sustained rebuild writes and the rebuild stalls.
• Failing replacement: a bad new drive (check its SMART data) can abort the rebuild.
• Replacement too small: it must be at least as large as the drive it replaces.
• Power interruption: an unclean shutdown mid-rebuild — a reason to run the NAS on a UPS.

What to do now

If the pool crashed, stop writing to it, do not reinitialize, and restore from backup (or consult professional recovery for critical data with no backup). Rebuild with healthy CMR drives, and adopt RAID 6 / SHR-2 plus a UPS to avoid a repeat. The table lists in-stock CMR drives.