Wii U Downloads will NOT store on SD Cards
Looks like USB drive is your only hope.
During an Q&A interview with Nintendo of Japan, details emerged about using external devices for storage purpose and while the translation could be off it detailed this.
- Downloaded software can go to internal memory or external USB device. SD Card not supported
- Your downloaded content cannot be transferred to another Wii U
- You can move content to USB drive but not copy
The details are still sketchy but it looks like USB flash drives and USB external hard drives are the way to go. Coming from someone who just bought a 32 GB SD Card, I'll have to wait till Black Friday to get a good deal on a 1 TB USB Drive.