the obvious questions are how big is your library, and how patient are you
writing 5Gb of
anything to a typical USB stick is 30 - 60 mins, and a cheap/small one will do its best to overheat and throw an IO error at about 4.9Gb
After that just be aware that almost every interaction - add, edit.. causes a rewrite of the same single metadata.db file and if that fails its bye buy library. So if you plan to edit or add say 10 books per day, that's ~3,500 rewrites of the same file
ask yourself - do you feel lucky.... or buy a pocket sized spinning drive, like the man said. Also, your average cat is less likely to mistake a HD for a mouse, then a mouse sized morsel of a USB stick. [ same goes for vacuum cleaners]