Symptoms
- RAID controller or NAS software reports 'degraded' status
- One drive's status light is off or red while others are normal
- Array still works but performance is worse than usual
- Notification or email alert about array health
Likely causes
- CommonOne physical drive in the array has failed or disconnectedThis is exactly what 'degraded' means: redundancy is gone but data usually survives.
- PossibleA drive was removed or a cable came looseSometimes it's a connection issue rather than true drive failure.
- PossibleA drive is throwing enough errors to be automatically dropped from the arrayThe controller protects the array by ejecting an unreliable drive.
What to do
- Identify exactly which drive is reported as failed or missing in the RAID/NAS management software.
- Check that drive's physical connection (power and data cables) before assuming it's dead.
- Do not remove or swap any other drives in the array while it's degraded.
- Back up the data on the array now if at all possible, since a degraded array has no redundancy left.
- Once backed up, replace the failed drive with a new one of equal or greater capacity.
- Start the rebuild process through the RAID/NAS software and avoid using the array heavily until it finishes.
- If a second drive fails during rebuild, stop and get professional RAID recovery help before doing anything else.
Tools and parts
Replacement hard drive matching array specsSearch: “NAS-rated hard drive same capacity”
When to call a pro
If a second drive fails during a rebuild, or the array won't rebuild cleanly, stop immediately and consult a RAID data recovery specialist rather than retrying.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.