Seagate 18gig Cheeta drives, model #ST318405 often have a firmware version (5063) which conflicts with Mac AV systems. Symptoms of the problem, include drive(s) not mounting or system not booting at all.
If the actual mechanism is visible (like in a hotswap drive), look for the firmware rev. 5063 is bad.
If the firmware version is not visible, call Glyph with the [glyph] serial number, and they will be able to determine whether the unit needs modification.

These must be modified _at_ Glyph.