Nintendo of Japan have issued some clarifications with regards to the storage options Wii U players will have when the console comes out.
"Downloaded software can only be played on the Wii U it was originally bought from. Meaning if you download a game to an external HDD and try to use that HDD on another Wii U, it won't work. Downloaded software is saved to the internal memory or what's in the USB slot. Downloaded software can't be saved to the SD card."