Battery-Powered USB Enclosure
An anonymous reader pointed us to a story about a battery-powered USB disk enclosure. It operates on AA batteries. It's aluminum, and is sorta meant to offload data from cameras. It's only 2.5 inches, so that's not totally unreasonable, but I'm still struggling a bit with the 'Why' part of the equation.
Could it be (and I'm stretching for the terms here) that this can act like a USB "master" (or host controller) and the iPod can't?
The last AA powered hard drive based MP3 player I had ran about 7 hours under continuous use(spinning up every minute or so). I would suppose that if the drive were spinning continuously 1.5 - 2 hours would be reasonable figure.