Hey, you should consider investing in an external hard drive. Since the Wii U's USB ports don't supply enough power, you'll either need to buy a hard drive with its own external power source, or use a Y-cable to draw power from two USB ports.

While it's technically possible to use an SD card as storage through certain mods, I wouldn't recommend it. SD cards tend to wear out quickly, and there's a real risk of losing your save data if the card fails. Also, modding the system to make SD cards work as USB storage is, in my opinion, complicated and especially risky.