Which exact model "powermac"?
Have you tried other ports?
Restarted the computer?
Tested other Firewire devices with your ports?
Used Disk Utility to verify the drive?
Reset PRAM?
[What to do if your computer won't recognize a FireWire or USB device|http://support.apple.com/kb/HT1317]
[abrody's Firewire Conundrums|Dealing with FireWire problems (drives not mounting, devices unrecognized, etc.)|http://reviews.cnet.com/8301-13727_7-10333969-263.html?tag=mfiredir] - Specific OSX version issues with specific drive versions.
[http://www.macmaps.com/firewirebug2.html]