Remove Missing Disk From Apple Raid

Just encountered this issue, ran into the same problem where I mistakenly added a disk to the mirrored RAID instead of just rebuilding using a replacement disk.

To remove the Missing/Damaged disk, you need to specify the RAID ownership of the disk you are trying to remove. So, in your case, you would enter:

diskutil appleRAID remove 8FEC705F-947F-4F84-AC8D-D5BE61F9037E disk5

(diskutil appleRAID remove *UID to remove* *RAID Device Node containing Missing/Damaged disk*)

Don't dwell on that issue. Just provide a replacement drive, add it to the RAID, and get it started on the rebuild.

Have you tried

diskutil ar remove

Usage: diskutil appleRAID remove MemberDeviceName|MemberUUID

RAIDSetVolumePath|RAIDSetDeviceName|RAIDSetUUID

Remove a member or spare from an existing RAID set.

Ownership of the affected disks is required.

Example: diskutil AppleRAID remove disk2s4 /Volumes/RAID

diskutil AppleRAID remove 11111111-2222-3333-4444-555555555555 disk5

Was repairMirror used to replace the original failed drive? If you use the add command, it is a different issue.

Usage: diskutil appleRAID repairMirror

RAIDSetDeviceName|RAIDSetUUID MountPoint|DiskIdentifier|DeviceNode

Repair a degraded mirror. The new member disk will replace the failed member.

Ownership of the affected disk is required.

'Add' added a new member, but the failed drive is still in a failed state. After the current rebuild is complete, remove one of the drives using 'remove'. Re-add the disk you pulled out back using repairMirror as a replacement. Unfortunately, it will do a full rebuild again during repair.

If you removed a disk from the RAID set but have not initialized it, you now have two standalone disks, which have the same data.

What is the output of

diskutil ar list

Backup one of the good disks using Time Machine, before we go further.