OS X :: Disk Ejects On Sleep And Won't Stop Spinning?

I replaced my superdisk with a 500 Gb Seagate drive in an optibay enclosure (unibody MBP running 10.6.2). The drive mounts fine on startup, and has not exhibited any unpredictable behavior in the file system, but it frequently ejects and provides me with this unhelpful message:The disk was not ejected properly. If possible, always eject a disk before unplugging it or turning it off. To eject a disk, select it in the Finder and choose File . Eject. The next time you connect the disk, Mac OS X will attempt to repair any damage to the information on the disk.

Once the drive ejects it is not visible in disk utility. I can hear it spinning continuously, but cannot access the drive. Moreover, I cannot access the drive unless I fully shut down the system and then reboot. A restart is often not sufficient to get the drive back up, but a full shutdown has worked every time so far.

The behavior seems to happen when the computer sleeps. I noticed that it would also happen sometimes if I handled the computer roughly by setting it down jarringly, so I took the whole thing apart, and reseated the drive within the enclosure, but the drive still ejects frequently, about once per day.