On a hunch I bought a 120 gb Western Digital Portable HD to try and see if I could hook it into my AV500 100 gb USB A port. I thought for sure the port would not have enough power to run the drive, but it does. I can play movies directly from the portable drive. I tried this out both on AC operation and battery and it worked well. I do think that the battery would be eaten up very quickly though.
I plan to load the 120 gb drive up with video and reserve the majority of the 100gb Archos drive for music files. This way I can take a large collection of video with me and play (with AC attached) most likely from hotels. I tried a file transfer from the portable HD to the Archos and it did OK, just very slow.
Here is a link to the drive at Target. It list for $179, but is on sale at $117 right now.
P.S. This forum seems to be dying. Not much activity.