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Have you got anything else that could be sapping power from your PowerBook? Is there anything plugged into the other ports (not just USB)? Is the PowerBook plugged into the wall?

Maybe repost your question and a solution here for future reference. :)
 
Fair enough.

The problem was that my external HDD wasn't showing up, just spinning and spinning and spinning. My PB was plugged in, the USB modem was in the other port. I tried turning the HDD off/on, unplugging it, eventually I opened it to see if it was plugged in properly, and before I even got it fully open it just appeared on my desktop. So really, it was less a case of "problem solved" more of "WTF?! It's working!"
 
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