“The 64DD sat under your N64. Well, not yours — you didn't have a 64DD.”
The Nintendo 64DD (Nintendo 64 Disk Drive) was a machine that like the Famicom Disk System, attached to a Nintendo 64 and could play games in a magnetic disk format, the disks containing their own internal memory, written through the N64 instead of the disk itself. There were also several peripherals used for it. It was released in Japan in December 1999. It was scheduled for release in America in 2000, but it ended up being a commercial failure due to the way it was sold (only through subscription to the RanDnetDD online service), so it was never released outside Japan. There were plans to bring it to international shores, however; on July 15, 2016, YouTube user MetalJesusRocks posted a video showcasing a then-recently-discovered prototype for an American 64DD with an included developers disk, both of which were verified as legitimate by a former Nintendo of America employee.
There were four Mario games released on this console, all in the Mario Artist series; there were also several cancelled Mario titles. Overall, there were only nine games released on the Nintendo 64DD, although other games planned for the console were released as Nintendo 64 cartridges instead. Only 15,000 of the 85,000 unsold units were melted for scrap.
Mario series games