I mentioned in a recent video that storage devices wouldn’t work with the Wii U if they had more than a 2TB capacity. Some people thought differently, but here’s the full quote from the official FAQ:
External storage devices used with Wii U should not exceed 2TB. External storage devices that exceed 2TB are currently not recognized by Wii U, but we are working to solve this issue via a December system update. Following the December system update the Wii U will be able to recognize a storage device that is over 2TB, but 2TB will still be the maximum amount of storage capacity. Any extra capacity in the storage device beyond the 2TB will not be accessible via the Wii U.
That should be enough storage space for the vast majority of people though.