alt Hacker NewsThe Disk System RAM Adapter plugged into the cartridge port on a Famicom. The Famicom 15 pin expansion port is just a fancy joystick port (the console is otherwise limited to a pair of hardwired joypads).
The NES expansion port on its own doesn’t seem to have enough lines to support the disk drive, notably it doesn’t have any address lines (CPU or PPU) at all, or the PPU data bus. It seems a bit weird frankly with the choices they made, I’m not sure what it’s actually for, apart from breaking the idea of extra sound chips in cartridges.
It was definitely for the disk drive. The idea was that you'd leave the disk drive plugged into the bottom of the system and then have a separate cartridge with the RAM, drive controller, and BIOS ROM. Somebody recently made a homebrew version that shows roughly how it would have worked, see here if you're interested: https://www.youtube.com/watch?v=0iyQ1N43U-U