Xbox 360 game disc layouts are optimized for the reading speeds of optical discs drives and attempt to concentrate the game disc data at the fastest read locations on the disc. This typically ends up creating gaps at the beginning, end, and in the middle of the disc layout where layer 1 transfers to layer 2. When we first released the “Play from hard drive” feature in 2008, we did not do any detection of gaps that can exist in between game code in disc layouts. We started our game copy at the first sector where the game started (skipping the gap at the beginning of the disc) and copied the disc until we hit the last readable sector of the disc where the game code ended (skipping the gap at the end of the disc). [....]
Fast forward to the 2009 release: we went back and examined how much potential space savings we could deliver to customers by removing these gaps that can occur in the middle of the game code in the disc layout.