All are grammatically correct 15:17 Oct 27, 2009
I've been sounding them out in my American English brain, and I can only hear slight differences in connotation. I would tend to use the first phrasing if the content of the disk were specified in more limited terms, i.e. "This disk only contains the primary OS," but it would be fine in a more general sense, too. The second phrasing seems to require a number, i.e. "The disk contains only one file." Your last phrasing sounds like the way a lawyer would say it -- thus Boris's example. | 
The translation workplace 
peer agreement (net): +2