ssscientist said:
You probably have your terms interchanged.
When you talk about a 'powered hard drive' that usually means an external adaptor or other power source, not your laptop. Most laptops don't have large enough power supplies to power anything but the internal components.
The term should have been "bus-powered" hard drive. While the MBP does have plenty of power budget for powering an external laptop-size hard drive, you probably want a desktop drive for performance reasons. No computer on the planet will power a desktop drive (31.2W) over FireWire, though it is within the theoretical specs.... You'd either have to use a PCI Express (PCIe) slot or more than one PCI slot to power such a FireWire card, though....
In fact, I once calculated that if you tied all the USB, FireWire, etc. ports on a typical laptop together, along with the CardBus slot, you would have almost enough power for the spindle start current on a full size desktop drive (without actually powering its circuit board).
There are bus-powered audio interfaces. M-Audio makes several. You probably can't realistically power that and an external HD, though. I've never tried, but I'd be pretty surprised.
The MOTU ultralite is a bus-powered interface. Remember that your battery life will be significantly shorter when powering an interface.
Also note that there are bus-powered laptop HD cases and there are bus-powered laptop HD cases. I have one that won't spin up unless the battery on my iBook is almost full and another one that works just fine.
Final note: there's no such thing as a bus-powered USB hard drive (or at least not with standard hard drives). USB doesn't provide enough power to spin up a hard drive (microdrives and possibly those little 1.4" PCMCIA-sized drives from an iPod notwithstanding). FireWire, yes. USB, no. (Well, there are some sort-of "bus-powered" USB hard drives, but they involve splitter cables and plugging into the PS/2 port or some other port for additional power.)