It's on the built-in bus in my G5 dually (rev.1) along with the boot drive. Occasionally, it just doesn't show up on boot up. It sometimes takes more than one re-start to get it mounted, then I may have no problems for weeks. It's a 250g Maxtor (Apple OEM - came installed when the machine was new.)

When mounted, it tests fine in Disk Utility and Disk Warrior, and seems to work properly (no other suspicious sypmtoms.) Could this be the sign of a drive gradually going bad?