The Wii U has an internal disc drive capable of reading both Wii discs and Wii U discs. It's believed to be somewhat related to Blu-Ray drives, in a similar fashion to the Wii's relationship with DVD. Like the Wii, the drive is slot loading and mechanically complex. The drive casing has a label indicating it was manufactured by "China Hualu Panasonic AVC Networks Co., Ltd.".
The drive is connected to the Wii U via a normal SATA controller, using AHCI 1.2. The IOSU may communicate with the controller (and, by extension, the drive), though it may need to encrypt the commands with the console-specific drive key from the SEEPROM before sending them[check]. The drive identifies as a standard SATAPI disk drive, connected to port 0. In vWii mode, the AHCI controller is placed into a special compatibility mode, which causes it to emulate the Wii's DI and provide accurate backwards-compatibility for Wii software.
Physically, the drive is connected with a custom 50mm ribbon cable, containing 28 pins at a 0.5mm pitch. The cable is non-standard - some pins are joined together, while others are disconnected. Notably, the cable is not symmetrical, which makes it vitally important that it is not put in backwards - the end with the extra tabs should connect to the disc drive, while the brown side of the cable should face away from the board underneath on both the motherboard and drive ends. The cable's pinout, when viewed from the drive end, black facing up, is as follows:
|Drive Cable Pinout|
Fail0verflow's Console Hacking 2013 article, specifically the "DI" section for information on vWii compatibility mode and AHCI.