Turns out AppleRAID is a lot like disksuite (except appleraid seems to drop disks out for no reason a lot of the time, and there’s no UI to tell you that happened).
Turns out you can ‘removefromraid’ one half of a mirror and the detached disk’s appleraid partition gets turned into HFS+ and mounted by the OS (great).
Problems happen when you removefromraid the remaining disk, mac os doesn’t manage to repartition properly (probably because I hadn’t unmounted the raid device).
imac:/Users/chrisl/ 1$ diskutil removefromraid 796AC197-9255-4747-AF1C-A62F05BA3451 B1EC6D40-F0BE-49EE-AAA5-945983DF5A65 Changing filesystem size on disk 'disk2s2'... Attempting to change filesystem size from 499763871744 to 499763888128 bytes The disk has been removed from the RAID
You end up with the partition having its ID left as ‘Apple_RAID_Offline’ and it’s not mountable and first aid refuses to try and fix it:
imac:/Users/chrisl/ 67$ gpt show disk8 start size index contents 0 1 PMBR 1 1 Pri GPT header 2 32 Pri GPT table 34 6 40 409600 1 GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B 409640 976101344 2 GPT part - 52414944-5F4F-11AA-AA11-00306543ECAC 976510984 262144 3 GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC 976773128 7 976773135 32 Sec GPT table 976773167 1 Sec GPT header
everything’s in index 2 – 52414944-5F4F-11AA-AA11-00306543ECAC. http://developer.apple.com/technotes/tn2006/tn2166.html tells me that’s Apple_RAID_Offline and that 48465300-0000-11AA-AA11-00306543ECAC is ‘Apple_HFS’.
Turns out you can delete and re-add the partition though since we still have the start and size:
imac:/Users/chrisl/ 3$ gpt remove -i 2 disk8 disk8s2 removed imac:/Users/chrisl/ 102$ gpt add -b 409640 -s 976101344 -t 48465300-0000-11AA-AA11-00306543ECAC disk8 disk8s2 added
and the OS will immediately mount the partition. This is handy if you formatted the drive you detached first without checking the other drive is okay to be used.