They can be hungry, can't they? I considered a pcmcia hdd for a long time before I stumbled across my 20gb jukebox. It may add another part I need to carry but the effect on my 6651 runtime is negligible. Plus it's no big deal to hang the jukebox off my belt and pop in the usb cable when needed. I shut the 6651 off if I just want music. Pretty good tradeoff IMO.
I don't know if I'll followup with my "external chassis" project considering so far the jukebox has met my needs. Maybe I'll put my 40gb drive in the jukebox and call it a day. Tho I like my chassis idea I may never get to it , time and moneywise