In the grand internet tradition of answering a loosely related question which is no use at all to the asker, I will say that the "smart kids" might use something like ZFS, which almost handles this for you. (Take snapshots, save delta streams on your USB stick. Requires the backup to be a ZFS copy, not just the same files.)
Useless right now at least. But I've been pretty happy with switching my storage to ZFS, even if the Linux version sucks. (I mostly don't use the Linux version.) I'd recommend it to anyone who doesn't mind a bit of transitional pain.