Not knowing anything about the purpose of what you're trying to do or your budget, and assuming the Pandora has only one or two USB ports, the following would seem like reasonable ideas:
1- use a fast, high capacity SDHC card instead of the HDD.
2- use a fast, high capacity CF card and a USB2.0 card reader.
3- use a USB flash drive.
4- using a HDD: the only portable HDD set up I've seen that reliably requires only one USB port is a 1.8" drive, most of which are low cap and run at 4200 rpm; most 2.5" HDD can power from a split USB cable, such that one USB plug works normally and the other just draws power. Now, I've definitely had the split USB cable not work even on desktop systems, probably because the USB bus wasn't getting enough juice to provide full power to both USB ports at the same time or something. However, if you can use a split cable, the power-only USB plug can run off of a standard USB wall adapter, just like you would think it would. So if you get something like a dual power iGo system, where you can power a USB device and the Pandora itself (with the right adapters), that would be a small set up to power the whole thing. Actually, since the Pandora can charge from USB, you would just need a powered 4-port USB hub to power the whole thing as well. Or, you could get a portable battery that has one or two USB ports.
No matter how you get some external memory storage to work, there is a danger that if you use a non-powered USB hub, there may not be sufficient power for both the ethernet USB adapter and the memory storage unit, even if you use a USB flash memory key. An alternative to getting an ethernet USB adapter is to get a portable wifi modem, which are popular with business travelers. Then you would be free to use wifi on the Pandora and hook up whatever external storage you can manage.
I think that's fairly comprehensive. I'd be curious to know if the new 2.5" SSD drives can run from one USB port or not.
I hope that helps.
