How to rebuild a software RAID mirror
Learn how to rebuild a software RAID mirror in Mac OS X.
When you disconnect a disk that’s a slice in a mirrored RAID set and then reconnect it, the disk needs to be rebuilt so it contains the latest data. If you previously enabled the "RAID Mirror AutoRebuild" option in the RAID options dialog when you created the RAID, the disk is rebuilt automatically for you in the background and you do not need to use the steps in this article.
If you did not enable the "RAID Mirror AutoRebuild" option in the RAID options dialog when you created the RAID, you need to rebuild the disk yourself. To rebuild a disk in a mirrored RAID set:
Open Disk Utility.
Select the RAID set, and then click RAID.
Click Rebuild.
Contact the vendor for additional information.