Family Computer Disk System

The Famicom Disk System is an accessory for the Famicom. It allowed the Famicom to play certain games on a floppy disk rather than a cartridge. Floppy disks had the advantages of being cheaper. Besides that, the disks were rewritable, making saving easier.

Sharp Corporation, a Japanese Electronics and Domestic Appliance company, created the Twin Famicom, a Famicom combined with a Famicom Disk System into one piece of hardware, but it was only released in Japan. It's unknown if Nintendo ever had plans to make an NES Disk System.

Games
Here is a list of Mario games released only for the Disk System.
 * All Night Nippon Super Mario Bros.
 * Famicom Grand Prix: F-1 Race
 * Famicom Grand Prix II: 3D Hot Rally‎
 * Golf: Japan Course
 * Golf: Japan Course (Gold)
 * Golf: US Course
 * Golf: US Course (Gold)
 * I am a Teacher: Super Mario Sweater
 * Return of Mario Bros.
 * Super Mario Bros.: The Lost Levels

Trivia

 * Mario and Luigi also appear in the Famicom Disk System BIOS. The Famicom Disk System BIOS also make a cameo appearance in Paper Mario: The Thousand-Year Door, on Sir Grodus' computer.
 * When sped up 16 times, the GameCube menu ambience is the startup tune for the disk system.