I use two externals on my X, but one I always use with power (i.e., the drive enclosure has the option to have it plugged in to the mains as well as USB). The other external is a little 1.8", which only uses USB power.
From what i've read elsewhere (a USB forum, I think), most USB hard drives must have power because the power output from USB 2.0 on a laptop is rarely sufficient.
Also, it could be enclosure(s). Cheap ones are just that: cheap.
