The Nintendo 64DD (Nintendo 64 Disk Drive) was an add-on for the Nintendo 64 released exclusively in Japan. Similarily to the much more successful Famicom Disk System, the Disk Drive used magnetic disks, offered large amounts of storage compared to most cartridges, and was plugged into the bottom of the console.
Plans were made to release the Disk Drive elsewhere, though after failing to make an impression in Japan the plans were cut. Its failure could perhaps be attributed to the lack of high-profile games for the hardware, its release late in the console's life, and the multitude of unincluded but previously announced features.
Some games previously announced with Disk Drive support were instead released without such support or were moved to competing consoles. For example, The Legend of Zelda: Ocarina of Time was originally planned to have expansion packs on the Disk Drive, but due to the failure of the Disk Drive, the expansion packs were never released.
The 64DD was discontinued in 2001. 
Only ten games were released for the add-on. They are: