I'm just too lazy to keep unplugging my 1TB external from my computer and vice versa. I have a 16GB thumb drive I picked up for $30 on Black Friday that I use, but that's also a bit of a pain in the ass.
Interestingly, some files (season 3 of "It's Always Sunny" that I ripped in exactly the same way as seasons 1 and 2, for example) always show up as "corrupted data" when I have my external plugged in, but are fine when they're on my thumb drive. I have no idea why.