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Fix a RAID that is degraded

We'll identify the failed drive, replace it with a compatible drive, and rebuild the array to restore redundancy—or guide you when recovery is not possible.

Troubleshooting · Home maintenance
1–24 hours (including rebuild)
What you'll need
  • Replacement drive (same size or larger, same type)
  • Access to RAID controller BIOS, NAS interface, or OS RAID tools
  • Screwdriver (if not using tool-less bays)
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Steps

Goal: Identify the failed drive, replace it, and rebuild the array to restore redundancy.

  • Check the RAID controller BIOS (during boot), NAS web interface, or OS tool for “Degraded” or “Failed” status.
  • Good: You see which drive has failed (slot or serial). Bad: Status unclear—check the manual or contact support.

Hardware RAID (controller or NAS)

Goal: Replace the failed drive and rebuild via the controller or NAS interface.

  • Identify the failed drive from the management interface. Power off the system (unless you have hot-swap bays).
  • Remove the failed drive and install a same-size or larger replacement in the same slot. Power on.
  • In the RAID interface, add the new drive to the array and start rebuild. Do not power off or remove other drives during rebuild.
  • Good: Rebuild starts and completes. Array shows Healthy. Bad: New drive not recognized—check compatibility or see Fix a disk that will not initialize.

Software RAID — Windows Storage Spaces

Goal: Replace the failed disk and repair the pool.

  • Open Settings > System > Storage > Manage Storage Spaces. Identify the failed physical disk.
  • Remove the failed disk (if the interface allows), add the replacement drive to the system, and use “Add drives” to add it to the pool.
  • The pool will repair automatically. Monitor progress in the Storage Spaces interface.
  • Good: Repair completes. Pool shows healthy. Bad: Repair fails—check that the replacement drive is initialized and compatible.

Software RAID — Linux mdadm

Goal: Fail and remove the bad drive, add the replacement, and let mdadm rebuild.

  • Run mdadm --detail /dev/mdX to see the failed device. If not already failed: mdadm --manage /dev/mdX --fail /dev/sdX.
  • Remove: mdadm --manage /dev/mdX --remove /dev/sdX. Add the replacement: mdadm --manage /dev/mdX --add /dev/sdY.
  • Rebuild starts automatically. Monitor with watch mdadm --detail /dev/mdX.
  • Good: Rebuild completes. Array shows clean. Bad: Rebuild fails—check dmesg for errors.

When to get help

  • A second drive failed before rebuild completed—array may be unrecoverable; restore from backup.
  • The replacement drive is not recognized—see Fix a disk that will not initialize.
  • Rebuild fails repeatedly or you need data recovery—contact a professional.

Verification

  • The array shows “Healthy” or “Optimal” in the RAID interface.
  • All volumes are accessible and data reads correctly.
  • No failed or missing drives are reported.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm degraded Identify which drive failed from the RAID interface.
  2. Replace drive Swap the failed drive with a same-size or larger replacement.
  3. Start rebuild Add the new drive to the array and start rebuild.
  4. Verify healthy Confirm the array shows Healthy or Optimal after rebuild.
  5. Call a pro Second drive failed, rebuild fails, or data recovery needed.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • RAID type and configuration (e.g. RAID 1, RAID 5)
  • Failed drive slot or serial number
  • Replacement drive model and size
  • Rebuild progress or error messages

Does the RAID show degraded or failed drive?

Check the RAID controller, NAS interface, or OS tool for array status.

Open the RAID management interface. Yes: One or more drives failed—proceed to identify. No: Array is healthy—no action needed, or a different problem (e.g. volume not mounting).

Yes No

You can change your answer later.

Volume not mounting?

If the array is healthy but you cannot access data, see Fix a volume that will not mount.

Done

Can you identify the failed drive?

The interface should show which slot or drive has failed.

Note the failed drive slot or serial number. Yes: Proceed to replace. No: Check the controller manual or NAS documentation for LED or slot indicators.

Yes No

You can change your answer later.

Replace the failed drive

Use a same-size or larger drive. Power off unless hot-swap.

Power off (or use hot-swap). Remove the failed drive, install the replacement. Power on. The controller should detect the new drive. Start rebuild from the interface. Good: Rebuild starts. Bad: New drive not recognized—check compatibility or see Fix a disk that will not initialize.

Yes No

You can change your answer later.

Wait for rebuild to complete

Do not power off or remove drives during rebuild. Rebuild can take hours. When complete, the array should show Healthy. Verify data is accessible.

Done

Second drive failed or rebuild failed?

If a second drive failed before rebuild, the array may be unrecoverable. Restore from backup. If the replacement drive is not recognized, see Fix a disk that will not initialize or check compatibility.

Done

Call a pro

If you cannot identify the failed drive, a second drive has failed, or rebuild fails repeatedly, contact a professional or restore from backup.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

What does "RAID degraded" mean?
One or more drives in the array have failed or been removed. The array is still readable but has no redundancy. Replace the failed drive and rebuild to restore protection.
Can I use a different size drive to replace a failed RAID drive?
The replacement drive must be the same size or larger. A smaller drive will not work. Same RPM and type (HDD vs SSD) is recommended for consistent performance.
How long does a RAID rebuild take?
Rebuild time depends on array size and drive speed. A 4 TB drive can take several hours to a day. Do not power off or remove drives during rebuild.

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