Losing firewire drives

Question

I have an early MacPro (the twin dual core 2.66) on which I use 2 external firewire drives fro backup.
One is a Seagate FreeAgent FW400 external drive, the other is a Maxtor OneTouch III FW800 drive. Both worked flawlessly until the Leopard upgrade. Now, however, they both just drop off at random - I'll come into the office in the morning and find a SuperDuper screen staring at me indicating that it can't find the target drive - the drives are no longer being seen by the machine.
The 2 drives don't drop at the same time (the FW800 seems to be more prone to getting lost than the FW400 drive). When they drop, they don't simply get unmounted, they don't show up in DiskUtility either. Powering down and restarting the drive generally brings it back to the desktop (I say generally, because while the FW400 drive always comes back, the FW800 won't necessarily behave in that fashion). Powering down and restarting the MacPro generally does NOT bring back the drives. Both drives are attached to the rear sockets on the MacPro, although the FW400 drive is run through the ACD port.
Any thoughts on a fix?

Answer 1

Odds are it's the enclosure and the chip used. The oxford 911 was the chip of choice for a long time. It may be old hat by now cause I use all esata drives now, but if you test the cables and it's not the cables, then odds are it's the enclosure. Run the mac Hardware Test (start from your CD/DVD and hold down the D key) and see if it by chance shows a failure. If not, try another enclosure that has an Oxford 911 bridge unless you learn there is another, newer, better bridge chip. I've bought literally hundreds of enclosures and the things I always checked were an active fan and an oxford chip. It may be that Leopard is involved but it's probably easier to get a better enclosure (assuming that is the issue) than abandon Leopard.