Incremental optimizations and modifications to a disk image can be made after the initial generation of the disk image. In the case of video games, publishers who have access only to a final game image can use the invention to make small improvements to the game disc image without the original game assets and without incurring the cost of generating an entirely new disc image. A mapping data structure represents physical locations of data on disk. The mapping data structure is modified with an editor, and the resulting modified layout is tested using an emulator that emulates interaction between a computer readable medium with the modified layout and computer hardware.