For 3 straight days, my 1TB external firewire 800 drive has turned itself off briefly and then come back on. Unfortunately, it is equivalent to me turning it off, and back on. Data could become corrupt.
I don't know what is causing this problem, and I don't know how to prevent it.
I have had the exact same problem, and it has been incredibly tense and frustrating to try to work.
However, I've found much more stability turning the Lacie drive on FIRST, and letting it run for awhile, before switching on the G5. I have done this a few times now, and the power has never switched itself off.
Anytime that I turn the drive on simultaneously with the G5, or post boot up, it turns itself off and stays off.
Hope this helps. I will continue searching for answers.