I get this question asked of me a lot recently via e-mail: What is an .MDS file when I download a game? How do I use it? Is it important, and so on.
This file is used by several different programs (such as DVD decrypter) . It generally is found with an .ISO CD image file and is the equivalent of what a .CUE file is to a .BIN image.
Wait, what is a .CUE file and a .BIN file? 😛 Ok, way back when when you needed to write a CD image file correctly, you would need to point your CD writing program to the .CUE file, so the program would know how to burn the .BIN file correctly. Think of it as a pointer, or table of contents that lays out how the .BIN file contents needs to be burn onto the blank media.
The .MDS file is similar to functionality to the .CUE file in that it lays out the table of contents so the .ISO or the .MDF (which is like an .ISO but different extension) can be burned correctly.
What does this all boil down to? Programs like Nero or DVD decrypter can burn the .ISO or .MDF file directly and you don’t need to worry about making a .CUE or a .MDS file. In all seriousness though, you don’t have to worry about it. By the time you tell nero to open up the .ISO file, you are almost done burning your data file. 😛