RAID Rebuild Failed on UGREEN NAS? What to Do Next

If the RAID rebuild has stopped, failed, or will not start, repeatedly clicking Repair can put additional stress on the remaining drives without fixing the underlying problem.

The first decision is whether the storage pool still has one recoverable drive problem or has entered a higher-risk state involving multiple drives.

TL;DR

• Do not retry a failed RAID rebuild until you know why it failed. First confirm the pool status, affected drives, replacement drive health, and whether the rebuild truly stopped rather than just progressing slowly.
• Retry is reasonable only when the pool is Degraded, exactly one required drive has failed, all surviving drives are healthy, and the replacement drive is healthy, unused, and at least as large as the failed drive.
• A rebuild can fail because of more than the replacement drive. Read errors on surviving RAID members, unstable drive connections, power loss, or additional pool damage can all stop the repair.
• Protect accessible data before another rebuild attempt. A degraded pool has reduced fault tolerance, and repeated rebuilds can place heavy read stress on the remaining drives.
• Stop rebuilding and move to data recovery if multiple drives are abnormal, the pool becomes inaccessible, the rebuild repeatedly fails at the same point, or irreplaceable data exists only on the affected RAID.

First, Check Whether the Rebuild Actually Failed

Open:

Storage > Storage Pool & Volume

Check the storage pool status and rebuild progress.

If the percentage is still increasing—even slowly—the rebuild may still be working. Large-capacity drives can require many hours to rebuild, especially when the NAS is handling other workloads at the same time.

If progress continues, do not cancel the process just because it appears slow. Use our guide to troubleshooting slow RAID synchronization instead.

Treat the rebuild as failed when one of the following occurs:

• UGOS Pro reports that the repair failed.
• The repair process stops and does not resume.
• The pool returns to Degraded after the repair attempt.
• The replacement drive disappears or changes to an abnormal status.
• The repair option is unavailable even though a replacement drive has been installed.
• The rebuild repeatedly stops at or near the same percentage.

Record the Current Storage State Before Making Changes

Before removing, moving, or testing drives, record:

• The storage pool status
• The RAID type
• The drive installed in each bay
• The status of every drive
• The failed rebuild percentage
• Any error message or notification
• Whether the replacement drive is detected
• Whether your shared folders are still accessible

Take screenshots if possible.

Do not change the bay order of the existing RAID members. Drive position information may be important if the pool later requires manual diagnosis or professional recovery.

If the files are still accessible, copy the most important data to an independent storage location before attempting another repair. A degraded RAID pool has already lost part of its fault tolerance.

Use the Pool Status to Decide What to Do Next

If you have not yet confirmed which disk caused the degraded state, start with the NAS drive failure troubleshooting guide.

Why a RAID Rebuild Can Fail

A rebuild does more than write data to the replacement drive. It must read the required data and parity information from the surviving RAID members.

The process can fail even when the new drive is healthy.

1. The Replacement Drive Is Not Suitable

The replacement drive should be:

• Healthy
• Detected consistently by the NAS
• Unused by another storage pool
• Equal to or larger than the drive it replaces
• Compatible with the NAS and intended workload

A drive with sufficient advertised capacity can still be slightly smaller than the original drive at the sector level. If UGOS Pro reports that the drive is too small, use a drive with greater capacity.

If the drive already contains partitions, storage-pool information, or another unsupported configuration, it may not appear as an available repair drive.

2. A Surviving RAID Member Has Read Errors

During a rebuild, the NAS must read large amounts of data from the remaining drives. A surviving drive may appear usable during normal file access but fail when the rebuild reaches a damaged or unreadable area.

A rebuild that repeatedly fails around the same percentage can indicate a read problem on one of the surviving members. It can also indicate a recurring connection or hardware-path problem, so the percentage alone does not identify the failed component.

3. The Replacement Drive Disconnects During Repair

An unstable connection can cause the replacement drive to disappear temporarily during the rebuild.

Possible signs include:

• The drive changes between normal and disconnected states.
• The drive is missing after the rebuild stops.
• UGOS Pro reports repeated drive insertion or removal events.
• The repair restarts from the beginning after the drive reappears.

Do not move surviving RAID members into different bays while investigating the replacement drive.

4. The NAS Loses Power or Shuts Down

A power interruption can stop the rebuild before it finishes. Depending on the pool state, UGOS Pro may resume the process or require another repair attempt.

Before retrying, confirm that:

• The NAS is receiving stable power.
• The power adapter and cable are secure.
• The NAS is not restarting unexpectedly.
• The system is not shutting down because of excessive temperature or another hardware condition.

For compatible models, a UGREEN NAS 120W DC UPS can add 0-second battery switchover and power-protection features that help reduce rebuild interruptions caused by a mains power failure.

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5. The Storage Pool Has More Damage Than Initially Reported

If another member develops errors during the rebuild, the repair may fail because there is no longer enough readable information to reconstruct the missing data. This is the point at which another repair attempt may increase risk rather than solve the problem.

Check the Replacement Drive

Open:

Storage > Hard Drive > HDD/SSD

Select the replacement drive and verify that it is:

• Detected
• Healthy
• Listed as Unused
• Not assigned to another pool
• Equal to or larger than the replaced drive

Run a Rapid test first.

If the Rapid test passes and the drive has no signs of physical failure, run a Complete test when time allows. A Complete test takes longer but checks more of the drive surface.

Do not use a replacement drive that:

• Fails either test
• Reports abnormal health information
• Disconnects during testing
• Produces clicking, grinding, or repeated spin-up sounds
• Is not detected consistently

If the replacement drive itself is faulty, replace it before attempting another RAID repair.

Check the Surviving RAID Members

Review every remaining drive under:

Storage > Hard Drive > HDD/SSD

Look for:

• Warning or abnormal health status
• Failed drive tests
• Increasing bad-sector or read-error indicators
• Unexpected disconnections
• Unusual mechanical noises
• A drive that disappears and reappears

If another RAID member is abnormal, do not begin another rebuild simply because the repair option is available.

A rebuild places sustained read activity on the surviving drives. Continuing with a questionable member can turn a degraded but accessible pool into an inaccessible one.

Confirm the Replacement Procedure

If the failed drive has not been physically replaced correctly, follow the NAS drive replacement guide before returning to the repair process.

The replacement must occupy the correct bay for the failed member. Do not rearrange the surviving drives to make the layout look sequential.

After installation, confirm that UGOS Pro recognizes the new drive as healthy and unused.

When It Is Reasonable to Retry the Repair

Retry the rebuild only when all of the following are true:

• The storage pool is Degraded, not damaged or inaccessible.
• Exactly one required RAID member has failed or is missing.
• All surviving RAID members show normal health.
• The replacement drive is healthy and consistently detected.
• The replacement drive is unused.
• Its capacity is equal to or larger than the failed drive.
• The NAS has stable power and is not restarting.
• Important accessible data has been copied elsewhere when possible.

To begin the repair, open:

Storage > Storage Pool & Volume

Select the degraded storage pool, choose the healthy unused drive, and use:

Use > Repair Storage Pool

Review the selected pool and drive carefully before confirming.

If Quick Repair is available for the storage pool, you may enable it to reduce the amount of data that must be reconstructed. If the option is not shown, continue with the standard repair process.

UGOS Pro may also provide Prioritize RAID synchronization. Enabling it can give the rebuild more system resources, but NAS performance may decrease until synchronization finishes.

Once the repair starts, avoid:

• Restarting or shutting down the NAS unnecessarily
• Removing any RAID member
• Changing drive positions
• Running heavy storage workloads
• Starting unrelated storage-pool operations
• Canceling the process because it appears temporarily slow

Monitor both the rebuild percentage and the health of every drive.

If the Repair Option Does Not Appear

If Repair Storage Pool is unavailable, verify:

1. The pool is in a repairable degraded state.
2. The replacement drive appears under HDD/SSD.
3. The replacement drive is healthy.
4. The drive is listed as unused.
5. The drive capacity is sufficient.
6. The pool has not lost more drives than its RAID level can tolerate.

Do not initialize the existing RAID members or create a new storage pool in an attempt to make the repair option appear. Those actions can overwrite storage metadata and complicate recovery.

If all requirements appear to be satisfied but the repair option remains unavailable, record the pool and drive information and contact UGREEN Support.

When to Stop Retrying

Stop repair attempts if:

• More than one RAID member is missing or abnormal.
• A surviving drive fails a health test.
• The pool becomes damaged, inaccessible, or uninitialized.
• The replacement drive repeatedly disconnects.
• The rebuild fails more than once after the original cause has been addressed.
• The rebuild repeatedly fails near the same percentage.
• A drive makes clicking, grinding, or other signs of physical failure.
• You no longer know the original bay order.
• The NAS contains irreplaceable data that exists nowhere else.

Do not:

• Initialize the original RAID members.
• Create a new pool with those drives.
• Move the surviving members between bays.
• Format a drive because UGOS Pro cannot mount the pool.
• Keep starting rebuilds without identifying why they fail.

If the pool is still accessible, copy critical files before doing anything else.

If it is inaccessible or multiple drives are involved, move from rebuild troubleshooting to NAS data recovery planning. For irreplaceable data or suspected physical damage, stop powering the affected drives and consult a qualified data recovery service.

After the Rebuild Finishes

When the repair reaches 100%, confirm that:

• The storage pool has returned to a normal state.
• All expected shared folders are accessible.
• Files can be opened correctly.
• Every RAID member is detected.
• No drive reports a warning or abnormal status.
• No new storage or drive alerts appear.

Run a Rapid test on the replacement drive and review the health status of the remaining members.

Then verify that important data is protected outside the RAID pool. RAID improves availability after certain drive failures, but it does not protect against deletion, corruption, malware, theft, or failure of the entire NAS. Use an independent NAS backup strategy for data you cannot afford to lose.

Frequently Asked Questions

Can I restart a failed RAID rebuild?

Yes, but only after identifying why the previous attempt failed. Confirm that the pool is still degraded and repairable, the surviving drives are healthy, and the replacement drive is healthy, unused, and large enough.

How long should a RAID rebuild take?

The time depends on drive capacity, RAID type, system workload, drive condition, and storage-pool usage. Rebuilding drives larger than 10TB can take 10–20 hours or longer. A rebuild that is progressing slowly is not necessarily failing.

Should I reboot the NAS after a rebuild failure?

Do not reboot automatically as a generic troubleshooting step. First record the pool and drive states. Reboot only when the system specifically requires it or when you are safely addressing a confirmed detection or hardware issue.

Can I use a larger drive as the replacement?

Yes. The replacement drive can be larger than the failed drive, but it must not be smaller. Additional capacity may not become usable automatically within the existing RAID configuration.

What if my files are still accessible?

Copy the most important files to independent storage before retrying the rebuild. A degraded pool has reduced or no remaining fault tolerance, depending on its RAID level.

What if the rebuild fails at the same percentage every time?

Repeated failure near the same percentage can indicate an unreadable area on a surviving drive or another recurring hardware problem. Stop repeated attempts, review every drive’s health, and protect any accessible data.

Is RAID repair the same as data recovery?

No. RAID repair reconstructs a missing member while the remaining RAID information is still sufficient. Data recovery is required when the pool is inaccessible, too many members are missing, storage metadata is damaged, or the remaining drives cannot be read reliably.