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I have LaCie 2big thunderbolt 2-disk device configured as a RAID1 from Disk Utility. One of the drives appears as "failed", so I unmounted the raid, replaced the drive and plugged it all back in.

Now Disk utility reports that one of the slices is missing from the RAID, and it shows my new disk separately on the list to the left.

The "slices", including the missing one, have a lock icon in front of them, and even after dragging my new disk to the list I can't rebuild the raid.

How do I rebuild the raid?

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3 Answers 3

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I was unable to find how to rebuild the RAID from disk utility after a couple of hours of research. However, I managed to do it from the command line.

sudo diskutil list  // to get a list of disks. Note the disk number of your new disk (e.g. disk4)
sudo diskutil appleraid list // note the uuid of the raid in question
sudo diskutil appleraid add member disk4 UUID-OF-YOUR-RAID

It will then kick you back to disk utility and start rebuilding the raid :)

Also, if the failed disk remains on the list, you can remove it:

diskutil appleraid remove UUID-OF-FAILED-DISK UUID-OF-RAID
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  • 1
    Thanks! It seems that the Raid GUI in Disk Utility is still broken.
    – Rob
    Feb 2, 2015 at 10:48
  • Had to do this on El Capitan, too. Coincidentally with LaCie 2big Thunderbolt (1st gen).
    – Jari Keinänen
    Nov 4, 2015 at 17:44
  • How long does the "add member" part take? I started this over an hour ago and it's stuck at 60%. The first bit ran quite quickly. I assume something isn't right, but there is a spinner on the left side of the percentage tracker that seems to indicate it's still working. Disk Utility and anything that uses that raid disk hang.
    – user632657
    Jul 8, 2016 at 1:10
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After plugged it all back in.

Disk utility reports that one of the slices is missing from the RAID, and it shows new disk separately on the list to the left.

You need to delete the missing disk from the right side screen and then click demote.

From the left side, drag and drop the new disk into the right then click on the new media and click rebuild.

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  • Going thru remove/minus first, then demote, before adding the replacement: this indeed appears to be the way to go. My degraded 10.9 Mini Server right now copies its 1TB to the new external drive, with the estimate starting at 19 hours. This should be the accepted answer as it uses the Disk Utility as of the original question, while the command line approach of course also is helpful. Thanks!
    – Dirk
    Jan 28, 2018 at 10:24
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You may not actually have a failed drive. If you had a power failure and you don't have all the external drives plugged into the UPS, external drives will be out of sync.

I have two 4 TB drives in my Mac Pro that are mirrored to a pair in external eSata enclosures. Power went off, and I discovered that I had plugged into the 'filtered, but not UPS' plugins on the UPS.

Rebuilding a 4 TB drive takes time -- I have both rebuilds going right now, and it looks like it will be about 12 hours to do them all.

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