Q: Powerbook Doesn't Recognize External Firewire

I've been using a Western Digital WD800 80gb HD in an external firewire/USB enclosure to back up my 17" Powerbook G4. Never had a problem with it. Recently, I took the WD800 out of the enclosure and installed a Western Digital 1600SB 160gb HD I took out of a windows machine.  For some reason, the Powerbook doesn't recognize the 1600SB in firewire.  Doesn't show up in Disk Utility.  However, when I connect it to the Powerbook via USB (the enclosure is Firewire/USB) it is recognized and shows up in Disk Utility.  I was able to format it that way. But, it still isn't recognized by the Powerbook when I switch back to firewire. 

When I revert back to the WD800 in the external enclosure, that drive works fine and is recognized in both FW and USB. So I know nothing's wrong with my FW port or cable.

Does anyone know why the Western Digital 1600SB cannot be recognized as an external FW drive ... but is recognized as an external USB?  Is there something specific about that particular HD?

My external firewire/USB enclosure has the Oxford 0x10483 software.

Thanks.