Wii U supports USB hard disk drives with a dedicated power source, called self-powered USB.
Flash memory devices have a limited number of rewrites and are not suited to long-term storage of important game data, so we do not recommend the use of flash memory devices as an external storage device solution. For best results, we recommend using a hard disk drive with its own power supply that connects to the Wii U via the USB port on the console.
The maximum amount of storage supported by Wii U is 2 TB (terabytes). Wii U will be able to recognise a storage device that is over 2 TB, but 2 TB will still be the maximum amount of storage capacity. Any extra capacity in the storage device beyond the 2 TB will not be accessible via Wii U.
To optimise performance and ensure a consistent experience, all USB storage devices must be formatted and configured for use with the Wii U console.
Please note that after formatting a USB storage device for your Wii U, it is formatted exclusively for that Wii U console, and cannot be shared as storage media with other devices such as personal computers or another Wii U console.