Just to add my bits to this. I use 2 Western Digital 500G MyBook's to store videos and audio on. I started out with them plugged into my Living Room MD, but I have moved it around and unplugged it and taken it to a friends house, to use his FIOS to download movies, and tunes, and then brought it back home, and plugged it in to any of my MD's and it has always came back after a few minutes or a quick reload router.
I have never lost any data, and I have done this several times.
So do not fear USB dirves. One thing I do, is as Thom suggests, use the pluto file structure, my videos are stored under /mnt/device/<number>/public/data/videos and the audio under /mnt/device/<number>/public/data/audio.
It is a wonderful thing. And, If you know someone else who has a LMCE system close by.....Sounds like movie night to me, grab your drive, and head over to a friends house.