I have a similar problem like a couple of people here, my drive will fall off the device list, and will only come back on when I do the following:
Eject tape (this is must, drive will not show if tape is not ejected)
Turn off drive
Turn on drive
The drive has fallen off two times this week, and it's been frustrating.
I'm running OS X Server 10.2.8, Retrospect 5.1.177, and firmware and drivers are up to date with the drive.