After getting the MacBook I reformatted the external drive to FAT32 so that I could talk to it from my XP box, my Ubuntu box, and the MacBook.
My old rsync setup failed me on Linux once I had switched to FAT32. I was using the "vaxE" option set and the "a" option (archive mode) was trying to preserve all permissions between the Linux files and the files on the external drive. Given that FAT32 doesn't support permissions, it was reporting chgrp permissions errors.
I changed my command switch to -rltDv, which works similar to -vax. Here's what each option does, basically:
r: copy recursively l: copy all links t: copy times D: preserve special files v: be verbose