Failed writeback to removable devices

External USB drives also have the advantage that they can be swapped out for offsite backup, without blowing your network usage cap trying to back up to some cloud service you don't control. (Also, you can be fairly sure you can get them *back* again, unlike some cloud service you don't control.)

Honestly, I suspect most of us here are doing last-ditch USB drive backups of *something*, at least.