Over the years, I've used a number of USB-powered drives.
They were all based on 2½ inch laptop drives, which run on 5 volts and use very little current.
Mine run fine on all my computers, Philips DVD player, Visionsat IV-200 FTA receiver, and Western Digital TV Live Plus media player.
Funny thing is, though... a buddy has one of the drives from my collection, and can't get it to run on his laptop without external power.
So, there is no guarantee.
Glad everything worked out so well for ya.
Wish I'd read this thread sooner - I might have gotten one, too.

Really hate having to find another AC outlet for the power supply, and remember to take it when I move the drive to my computer to extract the shows.