You can use the gui Disk Utility but it's not very intuitive. It's needs a repair button.
The manual command line way:
1. Open a root terminal command window.
2. Type and press enter to run:
diskutil list
The report will show three disks, probably "/dev/disk0", "/dev/disk1", and "/dev/disk2". The latter is not a real/physical harddrive, but is the mirrored set made up from the other two being mirrored. That one will have the name of your disk.
3. If that was "/dev/disk2", type the following:
diskutil checkRAID disk2
Under "status", you will see "degraded" and one of the two "slices" (under the # column - e.g. #0 or #1) will have "Failed", "Missing", or nothing (the latter, if erased or new drive) for a status code.
4. If disk0 had a status of "OK" and disk1 was the "bad" one; type the following command to repair/rebuild the RAID:
diskutil repairMirror disk2 1 disk0 disk1
Description of parts of this command:
repairMirror - this is verb - the action, we want to rebuild/repair a mirror
disk2 - this is the mirrored set/virtual drive that is the RAID
1 - this is the "slice", the drive # is step 3 above that was "bad"
disk0 - this is the "good" drive
disk1 - this is the "bad" drive being rebuilt.
This will like take an hour to several hours to complete. Check the status by typing:
diskutil checkRaid disk2
The previously "bad" slice will have a %number under it's status with the percentage done.
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