Update: I've decided to replace the entire thing, the chain of failures is getting to me. I've just spent another grand on parts, disks and controllers (online, of course), to replace the entire lot. I suspect this is a bad lot of disks. In the future, I am aiming for 4 disk redundancy, and no more than 4 disks from any manufacturer/range. I will be putting the old drives in the bin and passing on the RAID controllers. The new controller will be a 3ware 9550SX-16ML and it should arrive tomorrow (today?) along with another set of disks. Hopefully also today, I will be replacing the broken tape drive.