If they are big enough use them for backups. That is what I use mine for, with carbon copy cloner, after I put 500gb ssd's in the MBP's. If your router has USB you could connect one there for network attached storage.
Agree with post above about using them for backups, or perhaps for "scratch storage" -- stuff that isn't important enough to be on your internal drive, but that you'd just like to keep around.