https://twitter.com/RockstarSupport/status/379493174768439296

Rockstar Support
‏@RockstarSupport

@mfw For optimal performance, we recommend not installing that disc. We will have more info on our Support Site at launch.

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http://www.giantbomb.com/articles/xbox-360-owners-don-t-install-grand-theft-auto-v-s/1100-4735/

Some performance issues if you go beyond the mandatory install.

Installing and playing Xbox 360 games has become more complicated as this generation has gone on, especially when games ship on multiple discs. With Grand Theft Auto V, Rockstar Games is now warning players to avoid installing the game’s “play” disc.

The company issued the warning on its Rockstar Support account on Twitter, promising more details are coming soon.

GTA V comes with both an 8GB install disc and a separate “play” disc on Xbox 360. That initial 8GB install is required, and while you can optionally install the second “play” disc, Rockstar recommends against that for performance reasons. Once the mandatory install is over, the player does not have to ever swap discs.

It’s unclear if Rockstar can fix this issue with a patch or not. We’ll have to wait and see.

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here's a video detailing the performance issues:

http://www.youtube.com/watch?v=v0gPB0gdoOM

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pretty annoying. looks like you have to choose between dealing with some significant pop-in or the noise of the 360 fan