How to Rebuild a Degraded RAID Array on a Synology NAS
A degraded RAID array on a Synology NAS means that one or more drives are no longer contributing to data protection, but the volume may still be accessible. Synology Hybrid RAID (SHR), RAID 1, RAID 5, RAID 6, and RAID 10 can continue operating after certain disk failures, although performance and redundancy may be reduced.
The correct response is to identify the failed drive, protect the remaining data, and replace the damaged disk with a suitable model. Rebuilding a RAID volume is usually straightforward in DiskStation Manager (DSM), but an incorrect removal or an unsuitable replacement drive can make recovery more difficult.
Before touching the hardware, record the NAS model, storage pool status, drive serial numbers, and current alerts. The broader NAS storage guide can also help when comparing RAID layouts, drive capacities, SSD caching, and backup strategies for future upgrades.
What A Degraded Array Means
DSM marks a storage pool as degraded when the RAID system has lost a drive or detected a disk problem while the remaining members are still operating. The volume may remain available for file access, but it has less fault tolerance. A second failure could cause permanent data loss in RAID 5 or SHR-1, while RAID 6 and SHR-2 have greater protection.
A degraded state does not always prove that a drive has completely died. The cause may be unreadable sectors, repeated connection errors, a loose tray, outdated firmware, or a temporary communication problem. Check Storage Manager and the system notification panel before removing anything.
Open DSM and go to Storage Manager. Review Storage, HDD/SSD, and Overview pages for warnings. Select each drive to inspect its health information, temperature, bad-sector count, S.M.A.R.T. status, and connection history. The drive bay number shown in DSM should match the physical bay before you proceed.
Protect Data Before Starting The Repair
A RAID array is not a backup. It provides availability and, depending on the RAID level, protection from specific disk failures. It does not protect against accidental deletion, malware, file corruption, theft, fire, or a failed NAS enclosure. If the volume is still readable, copy critical files to another NAS, external disk, cloud storage, or a computer before rebuilding.
Avoid heavy file transfers, virtual machines, database workloads, media indexing, and large backup jobs during the repair. Rebuilding places sustained reading and writing pressure on every remaining drive. If another disk is already showing warnings, stop and create the most important backup possible before replacing anything.
Check the storage pool type and its repair requirements. A replacement disk normally must have at least the same usable capacity as the failed member. A disk marketed with the same nominal size may be slightly smaller because of differences in firmware or sector layout, so a drive with equal or greater capacity is safer.
Identify The Failed Drive And Replacement
In Storage Manager, select the degraded pool and open its drive details. DSM normally reports the failed bay and shows the drive as failed, crashed, missing, or critical. Compare the serial number and model displayed in DSM with the label on the physical disk. If the indicators do not agree, do not pull a drive based only on the bay light.
For hot-swappable Synology models, the failed disk can generally be removed while the NAS is running. Non-hot-swap models should be shut down according to Synology’s instructions. Use an appropriate NAS-rated hard drive or SSD, and confirm that its capacity, interface, and compatibility suit the enclosure.
The following summary shows how common configurations behave during a single-drive failure:
| RAID or SHR type | Usable protection | Single-drive replacement | Typical repair action |
|---|---|---|---|
| SHR or RAID 1 | One disk’s worth of redundancy | Usually supported | Replace the failed disk and repair the pool |
| SHR-2 or RAID 6 | Two disks’ worth of redundancy | Supported while one or two disks are unavailable | Replace the failed disk and monitor closely |
| RAID 5 | One disk’s worth of redundancy | Supported | Replace the failed disk and rebuild promptly |
| RAID 10 | Mirrored pairs | Depends on which member failed | Replace the failed disk if its mirror remains healthy |
| JBOD or Basic | No array-level redundancy | Not an array rebuild | Restore data from backups or repair the individual volume |
The exact behavior depends on the storage pool, volume arrangement, and DSM version. RAID 10 can remain operational after one disk fails, but a second failure in the same mirror pair may destroy that section of the array. Never assume that a visible volume means the data is fully protected.
Replace The Disk Safely
Power down the NAS if the model does not support hot swapping. If hot swapping is supported, confirm the failed bay once more and remove only that disk. Keep the failed drive labelled and stored safely until the repair is complete; it may contain useful diagnostic information, and replacing the wrong disk can turn a degraded pool into a failed one.
Install the new drive firmly in the correct tray and bay. Wait for DSM to detect it, then return to Storage Manager. The disk should appear as unused, initialized, or available for repair. If it does not appear, check the tray seating, power connections, compatibility list, and drive health.
Do not manually create a new volume on the replacement disk. The repair process must assign the disk to the existing storage pool. Formatting or using it as an independent volume can make the repair option unavailable and may lead to an unnecessary data migration.
Start The RAID Rebuild In DSM
Select Storage Pool in Storage Manager and choose the degraded pool. Click Action, Repair, or the equivalent repair command shown by your DSM version. Select the newly installed disk, confirm the warning, and start the operation. For some layouts, DSM may call the process “repair” rather than “rebuild,” but the purpose is the same: reconstruct the missing RAID data on the replacement drive.
The rebuild can take several hours or more than a day. The duration depends on disk capacity, RAID type, NAS processor, system workload, and the number of errors encountered. During this time, the volume may remain available, but applications can respond slowly and file transfers may have reduced speed.
Keep the NAS powered continuously during the repair. Connect it to an uninterruptible power supply where possible, and avoid forced shutdowns, firmware updates, or drive removal. A brief power interruption may pause the process, but repeated interruptions increase risk and can extend the rebuild.
Monitor Progress And Verify The Pool
Storage Manager displays rebuild progress, estimated time, and the current health of the storage pool. Check the status periodically, but do not repeatedly cancel and restart the operation. Review DSM notifications for new errors, elevated temperatures, bad sectors, or additional drives changing to warning status.
If the rebuild pauses or fails, record the exact DSM message before taking action. Do not initialize another drive or remove a second member simply because the progress appears slow. A support case with Synology may be safer than experimenting with the pool, especially when important files lack a current backup.
When the repair reaches 100 percent, confirm that the storage pool and volume show Healthy or Normal. Run extended S.M.A.R.T. tests on the replacement drive and review the other drives for warning signs. Check shared folders, permissions, packages, snapshots, and critical applications to ensure that the NAS is operating normally.
Reduce The Risk Of Another Failure
After the array returns to a healthy state, create or update a backup plan using the 3-2-1 approach: keep three copies of important data, use at least two different storage types, and maintain one copy away from the NAS. Hyper Backup, Active Backup, Snapshot Replication, and cloud synchronization can serve different workloads, but they should be tested rather than assumed to work.
Keep a record of the drive model, serial number, installation date, RAID type, and rebuild event. A clear maintenance history helps identify recurring disk or power problems. Even technical archives outside the storage field, such as this maintenance archive, illustrate why dated service records are valuable when equipment must be inspected or restored later.
Consider enabling health alerts, email notifications, and regular S.M.A.R.T. tests. Maintain adequate ventilation, avoid filling a storage pool to its limit, and replace drives that show repeated errors rather than waiting for complete failure. If the NAS supports it, a compatible UPS can protect the array from abrupt power loss during normal operation and during future rebuilds.
Practical Checks Before You Finish
- Confirm that the replacement disk is at least as large as the failed member and appears correctly in DSM.
- Verify that the storage pool reports healthy status after the repair reaches 100 percent.
- Run extended S.M.A.R.T. tests and review all existing disks for bad sectors or critical alerts.
- Test access to important shared folders, applications, snapshots, and backup jobs.
- Schedule a current backup and document the repair, drive serial number, and completion date.
A degraded RAID volume should be treated as a warning, not as a routine notification to ignore. Identify the correct bay, preserve the remaining data, install a suitable replacement, and allow DSM to complete the rebuild without unnecessary workload. Once the pool is healthy, strengthen the backup and monitoring plan so the next drive failure becomes a manageable maintenance event rather than an emergency. Review your Synology storage configuration today, verify that your backups can be restored, and replace any suspect drive before it creates a second failure.