The Nintendo 64DD is a rewritable disk drive and modem peripheral for the Nintendo 64. Released only in Japan via a mail order subscription, the system was first available in December 1999 for 12 monthly installments of 2,500 yen. The total package included the 64DD unit, a year of "Randnet" online service, and games released every two months. The drive fits underneath the N64 and connects via the EXT slot. The disks hold 64MB of data.
When it was originally announced in 1996, the Nintendo 64DD was touted as a way to bring persistent worlds and more interactive gameplay to the Nintendo 64. By the time it was released, the technology was outdated, and most of the games originally planned for the system, such as The Legend of Zelda: Ocarina of Time, had already been released as cartridges. One of Nintendo's more apparent failures, only eight titles were released for the Nintendo 64DD, and the online service lasted little more than a year before it was shut down. ~ Skyler Miller, All Game Guide