It was rumored that the Wii U would feature 8 GB of internal storage, but Satoru Iwata clarified that Nintendo did not reveal specifics regarding Wii U’s hard drive. Iwata reconfirmed that Wii U users can purchase a hard drive – of their choice – and attach it to the Wii U’s USB port. Although the Wii U may have a small amount of built-in storage, Nintendo may provide an SD card with each console – like they do with the Nintendo 3DS.
“We haven’t said anything about a hard drive that we’ll be including with the system itself. But we have said, through the USB connection, consumers will be able to take advantage of a wide range of the hard drives that are available on the market, and for quite cheap now, to add memory to their system however they like.”
Edit: The Wii U does have built in storage. Wii U uses an internal flash memory. It also supports SD memory cards and external USB storage.