Nintendo don't officially support SD/HC cards for Wii U downloads, however one Wii U user has found a nifty workaround.
Whilst a USB hard-drive is the official choice for running downloaded retail games or eShop titles, those who might have a spacious SD card lying around can "trick" the system by inserting it into a USB card reader. Simply pop in the SD card into the reader, and the reader's USB slot into the back of the Wii U.
When connected to the Wii U's USB port, the console assumes that the device is an external hard drive.
Nintendo do recommend using external hand-drives for a stable environment for downloads, and using SD card readers may be phased out with a future firmware update.