Repair a disk in a mirrored disk set
When you disconnect a disk that’s part of a mirrored disk set and then reconnect it, the disk needs to be repaired so it contains the latest data. If you selected the “Automatically rebuild” checkbox when creating the mirrored RAID set, the disk is repaired automatically whenever you disconnect and reconnect it. If you didn’t select the “Automatically rebuild” option, you can reconnect and repair a member disk manually.
Reconnect a disk that’s a member of a disk set.
Open Disk Utility, select the disk set in the sidebar, then click Repair.
Select the disk you want to repair, then click Choose.