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The Xbox 360 version of GTA5 comes on two discs. The "Install" disc (obviously!) is designed for you to install all of the relevant and important game files, and will speed up your experience playing the game once you get through the install. The other - Play - contains files that you'll only need to access while playing.
It might go against everything we've been told, but - apparently - installing the second disc of the game will actually decrease performance.
For optimal performance, we recommend only installing the Install Disc, and not also installing the Play Disc to the Xbox 360 Hard Drive or USB drive via Xbox Home (dashboard). You will be prompted to install the Install Disc the first time you boot the game.
The reasoning is unusual, but simple: GTA5 has been designed to stream content both from the HDD and the game disc, taking advantage of the way the Xbox 360 has been designed and gaining maximum speed - something that's impossible if all of the streaming data is kept in the same place.
The base game will still chew up 8GB of your Xbox 360 hard drive space - but, once it's installed, it'll run happily without any further interference.
Grand Theft Auto V is out right now for PS3 and Xbox 360 - and, of course, if you're playing the game on PS3, you can ignore all of this.