How to Rebuild a Synology RAID Array After a Disk Failure
A failed disk in a Synology NAS does not always mean that data has been lost. If the storage pool is protected by SHR, RAID 1, RAID 5, RAID 6, or RAID 10, the system may continue operating in a degraded state while you replace the failed drive. The priority is to identify the fault accurately, protect remaining data, and begin the correct repair process without changing the existing array structure.
Synology DSM makes RAID recovery relatively straightforward through Storage Manager, but the process still requires care. A rebuild can take many hours or several days, and the remaining disks experience sustained read activity during that time. A second failure can turn a manageable degraded array into a serious data-recovery problem.
The safest approach combines DSM status information, drive health checks, a current backup, and a replacement disk that meets the capacity and interface requirements. The exact menus can vary between DSM releases, but the underlying procedure remains similar.
Confirm The Failure Before Removing A Disk
Open Storage Manager and inspect the HDD/SSD and Storage sections. Look for a drive marked as Failed, Crashed, Warning, or Not Initialized. DSM may also report increased bad sectors, read errors, connection failures, or a storage pool status of Degraded. Record the drive bay number and serial number before touching any hardware.
A warning does not always mean that the disk has completely failed. A loose tray, damaged SATA connection, overheating, or a power problem can cause temporary disconnections. Review the system notifications and logs, then check whether the affected disk appears consistently offline. If it returns after a restart, do not immediately trust it as healthy; run an extended S.M.A.R.T. test and check its error history.
If DSM identifies a specific disk as failed, avoid repeatedly rebooting the NAS or moving drives between bays. RAID metadata depends on the existing disk arrangement, and unnecessary changes can make diagnosis more difficult. Photograph the drive labels and bay positions so that the failed unit can be identified correctly.
Protect Data Before Starting The Repair
RAID redundancy is not a substitute for a backup. Before rebuilding, confirm that important documents, photographs, databases, and application data exist on another device or in a suitable cloud service. If the NAS is still accessible, copy irreplaceable files first and verify that the backup can be opened.
If the NAS contains a mixture of business records and older technical archives, separate those backup jobs by importance and retention period. Operational documents, such as archived installation reports or maintenance records, should have an additional copy outside the array; a central maintenance reference can also help keep related records organized when equipment history matters.
Do not begin a rebuild if the NAS is showing signs of a second failing disk, repeated disconnections, unusual clicking, or rapidly increasing S.M.A.R.T. errors. In that situation, prioritize a sector-level backup or consult a data-recovery professional. Rebuilding a degraded RAID array is safest when every remaining disk is stable.
Select A Compatible Replacement Drive
The replacement drive must normally be at least as large as the smallest disk already participating in the storage pool. A disk advertised with the same capacity can be slightly smaller in actual usable sectors, so choosing a drive with equal nominal capacity is not always sufficient. A larger drive is usually the safer choice, although unused capacity may remain unavailable until the array is expanded.
Match the general drive type to the NAS and workload. NAS-rated hard disks are designed for continuous operation and vibration, while enterprise models may provide stronger error handling at a higher cost. SSDs should be selected according to Synology compatibility guidance, endurance ratings, and the requirements of any SSD cache. Do not mix an SSD replacement into a conventional hard-drive RAID group unless the storage design specifically supports it.
The table below summarizes the usual recovery position for common Synology layouts:
| Storage layout | Protection after one disk fails | Typical repair requirement | Important limitation |
|---|---|---|---|
| SHR or SHR-1 | Usually continues in degraded mode | Replace with a disk at least as large as the failed member | A second failure may cause data loss |
| RAID 1 | One mirrored copy remains available | Replace with equal or larger capacity | The array has no additional single-disk tolerance |
| RAID 5 | Parity rebuild is possible | Replace with a disk meeting the smallest-member size | A second disk failure during rebuild is critical |
| RAID 6 | Can tolerate two failed disks | Replace failed members one at a time | Rebuilds can be very long |
| RAID 10 | Depends on which mirror member failed | Replace the failed disk in its mirror pair | Two failures in one mirror can destroy the volume |
| Basic or JBOD | Usually no array-wide redundancy | Restore from backup or use recovery services | Replacing a disk does not rebuild lost data |
Replace The Failed Disk Carefully
If the NAS and drive tray support hot swapping, you can usually remove the failed disk while the system is running. Confirm the bay number in DSM before doing so. Pull out only the identified failed disk, label it, and keep it aside until the repair is complete. If the model does not support hot swapping, shut down the NAS according to Synology’s instructions.
Install the replacement disk in the same bay and wait for DSM to detect it. Do not use the new disk to create a separate volume, initialize it manually, or format it through another operating system. Those actions may not immediately erase the old array, but they add unnecessary risk and can make the correct disk-selection process less clear.
Return to Storage Manager, open Storage Pool, select the degraded pool, and choose Action followed by Repair, or the equivalent repair command shown by your DSM version. Select the newly installed disk when prompted. Read the capacity and warning information carefully before confirming, because repair permanently assigns the drive to the storage pool.
Monitor The RAID Rebuild
During a rebuild, DSM reconstructs missing data or parity onto the replacement disk. The pool may remain usable, but file transfers, media streaming, indexing, virtual machines, and package services can become slower. If the NAS supports scheduling controls, run the repair during a period of low activity while keeping the system powered and connected to reliable electricity.
Monitor the estimated progress in Storage Manager, but treat the time estimate as approximate. Rebuild duration depends on disk capacity, RAID level, disk speed, NAS processor performance, background services, and the amount of data already stored. A lightly used array may still require many hours because RAID recovery generally scans large portions of the member disks.
Avoid canceling the repair merely because progress appears slow. Also avoid removing another disk, changing RAID settings, or restarting the NAS unless DSM reports a problem. If the process stops, record the exact error message. A rebuild that repeatedly fails can indicate a defective replacement drive, another unstable member, insufficient capacity, or damaged RAID metadata.
Verify The Pool After Recovery
When DSM reports that the storage pool is Healthy or Normal, inspect every disk again. Run S.M.A.R.T. tests, review bad-sector counts, and check whether any drive has new warnings. A successful rebuild proves that redundancy was restored; it does not prove that every remaining disk is in good long-term condition.
Run a data scrub or consistency check if DSM offers that feature for the storage pool. Scrubbing reads the array and verifies parity or mirrored data, which can reveal latent errors that were not obvious during the initial repair. Schedule this when the NAS is lightly used, since it can create additional disk activity.
After the pool returns to normal, check shared folders, Synology applications, snapshots, and backup tasks. Confirm that permissions remain correct and that important files open normally. If the replaced disk is larger than the previous members, check whether the additional capacity is available; some arrays require a separate expansion step after all members meet the new size.
Build A Safer Recovery Routine
A disk failure is a useful signal to review the whole storage plan rather than simply replacing one component. Keep a written record of the NAS model, DSM version, RAID layout, disk serial numbers, backup destinations, and encryption keys. Store that information somewhere accessible even if the NAS is offline.
Use these practices to reduce the impact of the next failure:
- Maintain at least one current backup outside the Synology NAS and test file restoration periodically.
- Enable email or push notifications for disk errors, degraded pools, failed backups, and high temperatures.
- Replace disks showing persistent S.M.A.R.T. warnings instead of waiting for complete failure.
- Keep a tested spare drive available when the NAS contains business-critical data.
- Use a UPS and configure safe shutdown settings to reduce damage from power interruptions.
Consider whether the current RAID level still suits the workload. SHR is often convenient for mixed-capacity disks, while RAID 6 provides more tolerance for large arrays at the cost of usable capacity and write performance. RAID 10 can offer strong performance and quick mirror recovery, but its protection depends on the location of failures. The right choice should reflect capacity, backup speed, replacement availability, and the value of the stored data.
After replacing the disk and verifying the pool, open DSM and confirm that the backup schedule, notifications, and integrity checks are active. A repaired array is ready for normal use only when the storage pool is healthy, the files are verified, and an independent backup can be restored. Begin the repair from Storage Manager, document each step, and keep monitoring the NAS until the rebuilt array has completed its health checks.