RAID 5 Offline with One Failed Drive: What to Check First
RAID 5 normally remains accessible after one member fails. If the entire virtual disk is offline while the controller reports only one failed drive, another part of the storage state is preventing the controller from assembling a trustworthy volume.
Do not assume that replacing the reported drive and starting a rebuild is automatically safe. Record the complete member and controller state before another write occurs.
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
The reported failure may be accompanied by unreadable sectors or timeouts on a surviving member, an incomplete or foreign configuration, a controller or cache problem, lost communication through a cable or backplane, or a second drive that is present but no longer contributing correctly.
Controller labels summarize the decision the firmware has made. They do not establish that every online member is fully readable or that the displayed failed member is the only source of the outage.
What to record
- Controller, server or NAS model, firmware, and RAID level
- Virtual-disk and physical-disk status screens
- Slot, serial number, and status of every original member
- The first error, power event, reboot, replacement, or maintenance action
- Any rebuild, force-online, import, clear, initialization, or repair already attempted
What not to do
- Initialize or recreate the offline virtual disk
- Replace the reported member before identifying all drives
- Force a member online by trial and error
- Clear or import configuration without reviewing the complete set
- Run filesystem repair on a volume produced by uncertain member choices
How ADR evaluates it
ADR reviews the controller history, member identities, drive readability, and on-disk RAID metadata to determine why a nominally single-drive failure took the volume offline.
If another member is unstable, it is protected before reconstruction. Candidate RAID states are tested away from the original controller and validated at the filesystem and file level.
What happens next
An offline RAID 5 with one reported failed drive often retains recoverable data, but the unexpected loss of the whole volume is a reason to diagnose the surviving set before rebuilding.