Seeing your NAS report a degraded array is alarming, but in a redundant setup it is exactly the situation RAID was designed to survive. A degraded array means one drive has failed while your data remains intact on the others. The rebuild process restores redundancy onto a replacement drive. Handled calmly and correctly, you come out the other side with no data lost. Here is how to do it safely.
What degraded actually means
In RAID 1, RAID 5, RAID 6, or similar, the array can tolerate one (or two, for RAID 6) drive failures and keep running. When a drive fails, the array drops into degraded mode: still readable and writable, but with no remaining safety margin (in single-parity levels). Your priority is to restore redundancy quickly, because a second failure while degraded can lose everything.
| RAID level | Drives it can lose | Risk while degraded |
|---|---|---|
| RAID 1 | 1 (of 2) | Second failure = total loss |
| RAID 5 | 1 | Second failure = total loss |
| RAID 6 | 2 | Still protected against one more |
Step-by-step rebuild
1. Verify your backup first
Before touching anything, confirm you have a current backup. The rebuild stresses the surviving drives by reading every sector, and a marginal drive can fail under that load. If you do not have a backup and the data is critical, copy the most important files off the degraded array now, while it is still readable.
2. Identify the failed drive
Check the NAS dashboard to see which bay reports the failure, and note its serial number. Do not guess; pulling the wrong (healthy) drive from a degraded single-parity array causes immediate total loss. Match the serial number to the physical drive before removing it.
3. Replace with an equal or larger drive
Insert a replacement of the same capacity or larger, ideally the same or a compatible model. A smaller drive cannot join the array. If your unit is not hot-swap, power down before swapping.
4. Start and monitor the rebuild
The NAS usually detects the new drive and offers to repair or rebuild the array. Start it, then leave the system as idle as possible. Rebuild time scales with capacity, often many hours for large drives. Avoid heavy use and do not reboot mid-rebuild.
| Drive size | Approx. rebuild time |
|---|---|
| 2-4TB | ~3-8 hours |
| 8TB | ~8-20 hours |
| 12TB+ | Often 20+ hours |
After the rebuild
Once complete, the array returns to a healthy, protected state. Run an extended S.M.A.R.T. test on the remaining original drives, since one failure sometimes signals aging across a batch bought together. Confirm your alert emails are working so the next failure reaches you promptly.
If the rebuild fails
If a second drive throws errors during the rebuild, stop and rely on your backup rather than pushing a failing array harder. This scenario, an unreadable sector on a surviving drive during a RAID 5 rebuild, is precisely why backups and RAID 6 exist for larger arrays.
Preventing the next degraded event
A rebuild is a good moment to reduce the odds of the next failure catching you off guard. Drives bought together in one batch tend to age and fail around the same time, so if one goes, watch the others closely and consider staggering future replacements with drives from different batches. Turn on S.M.A.R.T. email alerts if you have not already, so the next failure reaches you the same day rather than sitting unnoticed in a dashboard. Schedule regular short and extended self-tests plus a periodic array scrub, which catches weak sectors before they surface during the stress of a rebuild. Keep the NAS in a cool, ventilated spot and clean dust from its fans, since heat is a quiet driver of premature drive failure. Most importantly, treat this incident as confirmation that your backup strategy is real: a rebuild that fails is survivable only if you have an independent copy of your data. If this degraded event made you nervous about not having one, fixing that gap is the single most valuable thing you can do next.
FAQ
Can I use the NAS while it rebuilds?
You can, since the array stays online, but heavy use slows the rebuild and adds stress to the surviving drives during their most vulnerable window. Keep usage light and let the rebuild finish as fast as possible.
Do I need the exact same drive model to rebuild?
Not exactly, but the replacement must be the same capacity or larger, and a similar performance class is best. Wildly mismatched drives can work but may reduce array performance. Matching the original model is the safest choice when available.
Bottom line
A degraded RAID array is a warning, not a disaster: your data survives on the remaining drives while you restore redundancy. Verify your backup, correctly identify and replace the failed drive, then start and patiently monitor the rebuild without heavy use. Finish by health-checking the other drives and confirming alerts work. Handled methodically, a rebuild is a routine recovery rather than a data-loss event.
