There is only one file type that uses the .NDR extension:
Nintendo 64DD Emulator ROM Save File
An NDR file contains game data saved by Project64, a program used to emulate games for the Nintendo 64 console and 64DD peripheral device only released in Japan. It stores the exact game data from the original 64DD disk and saved progress from gameplay in Project64.
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 provided several capabilities including support for 64MB magnetic disks for rewritable and expandable data storage. It was a commercial failure.
You most likely will only encounter an NDR 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. When you emulate an .NDD file in Project64, then save your progress as you play the game, an NDR file is saved alongside the NDD file to contain the saved game progress. When you load the game in Project64, the program checks to see if an NDR file is present, then loads any saved game progress.
Updated: March 10, 2019