Q We have hit a problem with our Mac mini Server, which has otherwise been proving an excellent purchase. We have it configured to use a RAID array, which we set up using Disk Utility. Before performing a major server configuration change, I broke the RAID mirror set to keep one copy as a fall-back in case the change went wrong. However it will not now start up from the remaining RAID mirror volume, although it did when the mirror was complete. Have I corrupted the mirror by breaking it?

A Software RAID systems such as AppleRAID (Disk Utility) and the third-party alternative SoftRAID rely on ‘boot-root’ partitions for each partition on each disk in a RAID array. The boot-root partition contains sufficient information to start up and load the RAID drivers, then hand over to the real root volume on the RAID.

Failure of this when trying to start up from one volume of a broken RAID is usually because these partitions were not properly built or updated, a not infrequent failing in AppleRAID. Repair is possible by rebuilding the boot cache, something that SoftRAID can perform from a menu command.

Alternatively, start up from another volume and do this for AppleRAID with the Terminal command
sudo kextcache -f -u /Volumes/volumename
where ‘volumename’ is the name of the volume that you are trying to start up from. This may be helpful with Mac mini Servers and some old hardware RAID cards which seem prone to the fault.

Updated from the original, which was first published in MacUser volume 26 issue 7, 2010.