How to Repair a Degraded Synology NAS Volume After a Drive Fails
Replace a failed drive and repair a degraded Synology storage pool in DSM 7 without losing data, step by step.
A "Storage Pool Degraded" warning in Synology DSM looks alarming, but it usually means exactly one thing: a single drive in a redundant array has failed or is failing, and your data is still intact and accessible. The array is running without its safety net, so the job now is to swap the bad drive and rebuild redundancy before a second drive goes.
Quick answer
A "Storage Pool Degraded" warning means one drive in a redundant array failed but your data is still intact. Confirm exactly which bay failed in Storage Manager (never pull a healthy drive), fit a replacement equal to or larger than the smallest drive in the pool, then use the pool's more menu and choose Repair. The rebuild can take hours to days, so keep the NAS powered and cool and avoid heavy writes. If the status reads "Crashed" rather than "Degraded," stop, redundancy is gone, so restore from backup instead of rebuilding.
Key takeaways
- "Degraded" means data is still readable; "Crashed" means redundancy is gone and data may be lost. Act fast while you are still only degraded.
- Repair only works on redundant arrays: RAID 1, 5, 6, 10, F1, or SHR with at least two drives.
- The replacement drive must be equal to or larger than the smallest drive in the pool.
- The Repair button only appears in Storage Manager while a pool is degraded.
- Rebuilds can take many hours to days; avoid heavy writes and do not power off mid-rebuild.
Degraded vs crashed: know which you have
The status word in Storage Manager decides everything. Read it before you touch any hardware.
| Status | What it means | Your move | Data risk |
|---|---|---|---|
| Degraded | One drive failed, redundancy reduced | Replace the drive and Repair now | Low, but no safety net |
| Critical (SHR-2 / RAID 6) | Two drives failed, last layer of redundancy | Replace and repair urgently | High |
| Crashed | Redundancy fully lost | Restore from backup, do not rebuild | Severe, possible loss |
| Healthy after repair | Redundancy restored | Run SMART tests, verify backup | Normal |
Only redundant arrays can be repaired at all: RAID 1, 5, 6, 10, F1, or SHR with at least two drives. A single-drive volume that fails has no redundancy to rebuild.
First, confirm which drive failed
Do not pull a drive until you know exactly which one is bad. Removing a healthy drive from an already degraded array can crash the whole pool.
- Open Storage Manager in DSM.
- Go to HDD/SSD and look for the drive marked Failed, Critical, or with a warning under Health Status.
- Note its bay number. Synology numbers bays left to right (or top to bottom on some models).
- Open Log Center and check for the SMART warning that names the failing slot.
Note
If two or more drives show as failed, stop and do not replace anything yet. A multi-drive failure can mean the pool has crashed rather than degraded, and the safe path is a professional recovery or a restore from backup, not a rebuild.
Get a compatible replacement drive
The replacement must be at least as large as the smallest drive currently in the pool. A larger drive works fine; the extra space simply will not be used until every drive in the pool is upgraded.
- Prefer a NAS-rated drive (Synology, WD Red, Seagate IronWolf) over a desktop drive.
- Check the Synology compatibility list for your model if you want a fully supported configuration.
- A brand-new drive is ideal. Avoid reusing a drive that may also be near end of life.
Replace the drive
Many Synology models support hot-swapping, meaning you can replace a drive while the NAS is running. Models without hot-swap support must be powered off first.
- If your model does not support hot-swap, shut down the NAS cleanly from DSM then Shutdown.
- Slide out the tray for the failed bay you identified earlier.
- Unscrew or unclip the old drive and mount the replacement in the tray.
- Slide the tray back in until it seats firmly. If powered off, boot the NAS back up.
Run the repair
With the new drive installed, DSM exposes the Repair action on the degraded pool.
- In Storage Manager, select the degraded storage pool.
- Click the (more) icon in the upper right and choose Repair.
- Pick the newly installed drive as the replacement when prompted.
- Confirm. The pool status changes to Repairing and a progress percentage appears.
If Fast Repair is offered, you can enable it to skip rebuilding unused regions and speed things up. If you previously assigned a hot spare and enabled Auto Repair, DSM may have already started rebuilding onto the spare without your input.
A word on what the rebuild is actually doing, because it explains why it takes so long and why it is risky. To restore redundancy, DSM reads every block from the surviving drives, recalculates the parity or mirror data, and writes the reconstructed data onto the new drive. On a parity array like SHR or RAID 5, that means reading the entire surface of every remaining disk end to end. A worn drive that has been quietly limping along can fail under that sustained read load, which is exactly why a current backup matters most during this window. The bigger the drives, the longer the exposure: an 8TB or larger array can keep every disk pinned at full read for a day or more. Avoid scrubbing, heavy transfers, or surveillance recording until the status returns to Healthy.
Note
Keep the NAS powered and cool during the rebuild. A rebuild forces every remaining drive to read end to end, which is exactly when a second marginal drive is most likely to fail. This is the riskiest window, so make sure your backup is current before and during the process.
After the repair finishes
When the status returns to Healthy, the array has its redundancy back. Take two follow-up steps:
- Run an extended SMART test on every remaining drive to catch any other drive that is borderline.
- Confirm your backup job ran. A NAS is not a backup by itself; pair it with the principles in our guide to ransomware-proof 3-2-1-1-0 backups.
What to do right now
If you just saw the degraded warning, work through this in order and do not rush the hardware swap:
- Open Storage Manager and read the exact status word: Degraded, Critical, or Crashed.
- If two or more drives show failed or the pool says Crashed, stop and restore from backup instead.
- Identify the failed bay number under HDD/SSD and cross-check it in Log Center.
- Confirm your latest backup is current before touching anything, the rebuild is the riskiest window.
- Fit a NAS-rated replacement at least as large as the smallest drive in the pool.
- Select the degraded pool, open the more menu, choose Repair, and pick the new drive.
- Keep the NAS powered, cool, and lightly loaded until the status returns to Healthy.
Frequently asked questions
How long does a Synology rebuild take?
Anywhere from a few hours to several days, depending on drive size, RAID type, and how busy the NAS is. A multi-terabyte SHR-2 rebuild on a loaded system can run well past 24 hours.
Can I use the NAS while it repairs?
Yes, the data stays accessible, but performance drops and heavy writes slow the rebuild. Light use is fine; avoid large transfers until it finishes.
My pool says "Crashed," not "Degraded." Is the Repair button gone?
Yes. A crashed pool has lost its redundancy entirely, so DSM cannot rebuild it. Restore from backup, or consult a data-recovery specialist before writing anything new to those drives.
Why does the new drive show as "Not Initialized"?
That is normal for a fresh drive. The repair process initializes and adds it to the pool automatically once you select it during the Repair step.
