Due to my Terastation being at the time in an isolated location I set it up with RAID1 plus two hot spares, so that (theoretically) it could survive two drive failures without losing redundancy.
Unfortunately, when a drive failed it just sat there, degraded. I was unable to convince it to use the hot spares (in fact they had vanished from the RAID array window). I got to the physical drive and replaced the bad drive and hit the function button. It formatted the replaced drive as a normal disk, but didn't give me the opportunity to rebuild the raid. I now had 1 normal drive, one drive that was in a degraded RAID array, and two hot spares that weren't in any array but couldn't be reformatted or removed.
I noticed that the new drive could now be removed removed the new drive, reseated it, rediscovered, and it finally let me rebuild the array with it.
Then on a hunch I reseated the two hot spares and they came back as "spare disk" members of the RAID array.
At this point I am somewhat concerned about the Terastation. Is the problem that only RAID5 will use a hot spare, and I should burn the current config to the ground and go with a 3 drive RAID5 and a hot spare... or is this just one of those things, it is what it is, and build me a FreeNAS?