Family Computer Disk System

The Family Computer Disk System, also called the Famicom Disk System, is an accessory for the Family Computer, released in Japan on February 21, 1986. It allows the Family Computer to play certain games on a proprietary floppy disk format called "Disk Cards" rather than ROM cartridges. Disk Cards had the advantages of being cheaper and allowing for higher data capacity than cartridges at the time. Besides that, the disks were rewritable, making saving easier. Nintendo sought to make Disk Cards a permanent standard for all of their future games at that point, though this plan was abandoned a few years later, due to various issues. The Famicom Disk System has a yellow character named Diskun as its mascot.

In 1986, Sharp Corporation released the Twin Famicom, which combines the Family Computer and the Family Computer Disk System into one piece of hardware, and it was also only released in Japan.

BIOS
Mario and Luigi appear in the Famicom Disk System's BIOS, which is shown whenever the device is booted up. There, Luigi turns off the light, only for Mario to turn it back on again. This sequence loops, with a different color being displayed each time, until the player inserts a disk. Mario has a similar palette as in Donkey Kong while Luigi's palette is the same as in Super Mario Bros. The Twin Famicom's BIOS is nearly identical except the Nintendo logo instead reads "FAMICOM".

Issues
The main reason why Family Computer Disk System was not released outside Japan and why it eventually lost developer support altogether is believed to be due to a lack of success caused by various issues with the system:
 * The games were significantly easier to pirate; the way the Disk System recognized pirated games was by checking to see if the "I" and the third "N" in the embossed "NINTENDO" logo at the bottom of a Disk Card was present, via fitting a raised version of the same logo into the embossing. The intention behind this was for Nintendo to be able to directly counter production of unauthorized Disk Drive games by suing their manufacturers for trademark infringement, assuming that pirates would copy the full logo outright. However, it was easy for pirates to bypass this check without directly copying Nintendo's logo by creating alternate off-brand logos with the "I" and third "N" intact (i.e. "NINFENDO" or "NINIENDO") or by simply embossing blank spaces that occupied the locations of the necessary letters. Furthermore, the fact that Disk Cards were simply modified versions of Quick Disks (a brand of floppy disk easily purchasable in stores in the late 1980's) meant that unauthorized Disk Cards could be easily manufactured by attaching a copy check-compatible piece of plastic to the bottom of a Quick Disk.
 * The games were easier to damage; Disk Cards, being a form of magnetic media, were sensitive to magnetic wavelengths, and unlike regular 3.5-inch floppy disks, most Disk Cards did not include a shutter to protect the window that exposed the magnetic disk inside; shutters were only included on blue competition cards and gold prize cards. The lack of a shutter meant that the disk could get scratched, dirty, or even grow mold in severe cases.
 * The system itself was more fragile than the base Famicom, due to the large number of moving parts needed for the disk drive; in particular, the rubber belt that the system used was prone to wearing down much faster than that of a standard floppy disk drive.
 * Any games that made use of the supplementary audio channel provided by the Disk System's 2C33 chip would have to be drastically altered during localization for international markets to conform to the audio capabilities of the standalone Famicom, as no devices were released for the Nintendo Entertainment System that supported 2C33 chip audio.
 * The games had lengthy loading times at various points (often when swapping sides, or when entering particularly data-heavy areas). This is because magnetic disk drives have to physically seek out where data is located on the disk in order to load it into RAM. By comparison, cartridges are able to load near-instantaneously due to all the data being stored in a single location, that being the on-board ROM chip.
 * Most games would have to be split across both sides of the Disk Card due to the small size of the magnetic disks; typically, the first and final portions of the game would be stored on Side A, while the rest would be stored on Side B. Consequently, whenever the player progressed to a certain point in a game, they would be required to eject and flip the Disk Card before reinserting it.
 * The jewel cases that contained the games were smaller than cartridge boxes, and were therefore easier to overlook in stores or lose in homes. The cases were also required to fully protect the Disk Card, whereas cartridges could be stored, standalone, on shelves.
 * The technological superiority of the Disk Card format was short-lived, with higher-capacity cartridges becoming cheaper to produce just a few years later. Combined with the higher rate of piracy that Disk Cards suffered from, this deprived the format of the practicality that served as its primary selling point for developers.

The piracy issue was an especially big problem for Nintendo, and is believed to be the source of their stringent policies regarding copyright protection. It is also widely believed that the Disk System's high piracy rate is what convinced Nintendo to use cartridges for the Nintendo 64 rather than the technologically superior optical discs seen in their rivals, the Sega Saturn and Sony PlayStation.

In 1986, Nintendo sought to counter Disk System piracy by installing special Disk Writer kiosks that would allow consumers to download games onto Disk Cards for ¥500 as opposed to the retail price of ¥2,600; some Disk System games were even exclusive to these kiosks. The service was very popular, remaining in place until the Family Computer line's discontinuation in 2003, nine years after the Family Computer Disk System itself was discontinued.

Trivia

 * The BIOS makes a cameo appearance in Paper Mario: The Thousand-Year Door, on Sir Grodus's computer. His computer accepts Data Disks that resemble a Disk Card.
 * When sped up about 16 times, the Nintendo GameCube menu ambience is revealed to be a slowed-down version of the startup tune for the Disk System BIOS.