So I guess I'll either have to plug one or more drives into an external AC adapter or power on an actual desktop PC to copy files between these two drives. It seems my 3A (I think) AC adapter just can't handle these both at once. Which... you know, that's totally fair. I didn't think I'd ever hit the limit but here we are.
@noiob I would probably do a low-capacity desktop drive either in an adapter or just an old external. (I'm sure desktop 1TB externals aren't made by now...)
@noiob I've heard about that working for the Wii U, but eh, I'd play it safe.