The only disk drives Apple will sell us are 250GB and 500GB, which seems a little excessive for a scratch disk. Apparently Seagate has a 750 GB drive, but that's moving in the wrong direction.
I'm looking for a drive in the 40-100 GB range - preferably something cheap. Is any old desktop disk drive compatible with the Mac Pro? The small amount of information I've found so far implies that a Mac Pro can use any SATA drive you give it...