Windows "main.boot.disk" folder in windows 2003 SBS

I recently got my hands on a PC with windows 2003 SBS installed. I'm only using it to test around things (will get reformatted at some point). Looking around I found a folder named "main.boot.disk" in the root folder of the main disk. This "main.boot.disk" folder seems to have a copy/mirror of another (?) W2003 hard disk, but I have never seen such a folder, eventhough it look like a system folder.

Googling, even with quotes around the folder name, does not produce usefull results as the terms are too generic.

Does anybody know what this folder is, what is its purpose?