I have a g-drive connected through FW800 (Powered through FW800 also), it houses only my iphoto database. Every two to three days, the drive unmounts and I need to turn it off and turn it back on.
Have you verified it or possibly repaired it in disk utility?
Tried another FW800 or power cord if possible?
I have this same problem from time to time with my 800 external and I relate it either to no being ejected properly or being shut off and on constantly.