It should. Already works with the Wii.
Wait, this trick won't work with the Wii U because the FAT32 format doesn't support files bigger than 4GB. Nintendo might get around this limitation by splitting their backup format but I really doubt they'll do something like that.
Thanks. Maybe won't have to reformat on Wii U? It's designed to work with external HDD/SDD so hopefully will work?
Really don't want an HDD sat next to my WiiU