There is only one file type that uses the .NDD extension:
Nintendo 64DD Emulator ROM File
An NDD file contains a Nintendo 64 (N64) game created for 64DD, a disk drive peripheral for the Nintendo 64 game console only released in Japan. It stores the game data from the original 64DD disk that can be run on a computer using an N64 64DD emulation program.
The 64DD device was released in 1999 in Japan as a peripheral that attached to the bottom of the N64 console through the extension port. It extended the functionality of the N64 to use 64MB magnetic disks for rewritable and expandable data storage and allow users to create movies, animations, and characters that could be shared online. It was a commercial failure.
You most likely will only encounter an NDD file if you are a gamer who enjoys emulating old games on your computer and is looking to play a game from a 64DD disk. Project64 can be used in Windows to emulate NDD files.
NOTE: NDD files are often called 64DD game ROMs or game image files.
Updated: April 4, 2019