You can expand the amount of available storage space with external USB storage devices.
Nintendo recommends using self-powered hard disk drives, i.e. devices that use an external power supply instead of drawing power through USB. Devices that draw their power from USB ports are known as "bus-powered" devices, and are not recommended for use with Wii U. For the best results with storage devices, we recommend using a hard drive with its own power supply that connects to the Wii U via the USB port on the console.