Most external drives (even notebook, 2.5" size, ones) will need external power. Anything above a 2.5" notebook drive uses 12V as well as 5V. There is usually no access to +12V on any external ports. Some notebook drive size external drive kits have connectors to two external jacks to get up to twice the 5V current of a single port to run the drive, but often even that isn't enough. I recommend getting one with an external power supply.
. A kit made from a notebook drive could be powered by a battery pack made up of four 2000mAhr or higher AA batteries so it could still be truly portable.
http://www.compgeeks.com has a USB battery pack (designed to power all that junk you can get to plug into a USB port: mini-fan, toothbrush, "personal massager", night light, etc. etc.) that could be modified for this purpose.
.bh.