Also known as FDS
The Famicom Disk System was a 1986 Japan-only add-on that swapped cartridges for rewritable disks, adding save support, an extra sound channel and cheap re-writes at in-store kiosks. It sold millions of units before falling cartridge prices made it obsolete.
Several landmark titles debuted here before their cartridge versions: the original Japanese releases of The Legend of Zelda, Metroid and Castlevania, plus Doki Doki Panic — the game reworked into Super Mario Bros. 2 for the West.
Disk titles have an extra FM sound channel and built-in saving, so the Disk System versions often sound different from the cartridge releases you may know.
Games spanned both sides of a disk and prompted players to flip it. The player handles the swap for you when a game asks for the other side.