Without thinking about it too hard, doing a backup that way doesn't make a lot of sense anyway. You're liable to get a lot of "blank" space taken up by unfilled filesystems, and it requires a restore to a drive of the exact same size (unless you don't care about your new 120GB drive still only having enough space to store 12GB worth of music). It makes more sense to back up filesystems as files and only read raw devices for non-filesystem slices.