make sure you have the latest firewire as well as firmware updates as through the software update control panel.
definately zap your pram as it locked up my firewire burner on my pismo powerbook. APPLE + OPTION + P + R
I've also found that 9.1 seems to be more firewire friendly than 9.0.4. to be quite honest, i couldn't stand it when it came shipeed with my 'book. this may get rid of your problem.
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"the computer for the rest of us...." -SJ