I've got the original Toshiba Thrive...they've been frequently on sale in refurb for $199 lately if you have some patience. I got my 8GB on Black Friday for $199 new from Newegg. Full size USB. It's good for this kind of thing.
Keep in mind the HDD will have to be formatted FAT32 for android (although there is an NTFS mounting APK available on thriveforums.org for a rooted Thrive).
Archos - they make mediocre tablets mostly, though, compared to the competition, but one of the pros is that they tend to bundle a lot more video/audio codecs (so Archos tabs can generally play a lot more formats).