While the Wii U only comes with 8 gigabytes or 32 gigabytes of internal storage depending on the version, Nintendo is allowing users to store downloaded games on external devices. However, those external devices are limited to USB flash drive and hard drives according to Nintendo’s Q&A section on the console. The SD card cannot be used to hold any downloaded software.
Additionally, downloaded games are tied to the console they are purchased on and cannot be played on another Wii U. That means that you will not be able to take the USB flash drive holding the Wii U games you purchased and put it in someone else’s Wii U and play your games. Nor will you be able to re-download games you purchased on someone else’s Wii U and play them with your account like you can with the PS3 and Xbox 360.
I wonder if your Nintendo Network ID can be moved around from console to console like the PS3 and Xbox 360 or if it is locked to a single Wii U.