What to do:
1. Reinsert the SD card
a. Make sure the Wii U is off and remove the SD card.
Check that the SD Card is compatible.
SD memory cards up to 2 GB
SDHC memory cards up to 32 GB
SDXC and fast-transfer UHS-type memory cards are not compatible.
b. Check the SD card and ensure it is unlocked.
c. Re-insert the SD card and turn the system on.
2. If you are comfortable with deleting the data on the SD card, try reformatting it.
3. Try another SD card or try the SD card in another device, such as a PC or other compatible device.
If your problem is still not resolved, please contact Nintendo Consumer Support.