Well turns out the 0:7 disk was somewhere else and 0:0:3 just hadn't powered up for some reason and nobody noticed until I took over (fresh eyes maybe?). Anyway, unplugged the drive and plugged it back in and it's now added to the RAID and rebuilding.
I have two MD1200 enclosures connected to a PERC H800 adapter (unified mode). Each has 12 SAS drives. The drives have IDs of 0:0:0 thru 0:0:11 and 0:1:0 thru 0:1:11 as expected. Except for the 4th drive in the 1st enclosure which has ID 0:7 and shows up under Physical Disks outside of the two enclosures (red arrow in image). [IMAGE] The drive is Ready and Spun Up with a green tick. The only available tasks are "Assign Global Hot Spare" and "Clear ...". It is the same model and revision as other drives in the enclosure. If I run reports on the enclosure, there is no 0:0:3 disk and the Slot Occupancy Report shows an empty slot. Firmware version is 12.10.7 and drive firmware etc is latest. I inherited the system like this and I believe it has always been like it since installation. Any thoughts on why this would happen or how to fix it?
Disk not shown with Enclosure Physical disks and has unusual disk ID