Manually repair a disk in a mirrored disk set using Disk Utility on Mac
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” tickbox when you created 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 disk member manually.
Reconnect a disk that’s a member of a disk set.
In the Disk Utility app on your Mac, select the disk set in the sidebar, then click Repair.
Select the disk you want to repair, then click Choose.