Grand Theft Auto V comes with two discs on Xbox 360, but you should only install one of them.
The first disc requires a mandatory 8GB install to either a HDD or USB stick, and then you'll never need it again.
The second disc labelled "Play" is used to actually run the game, but should NOT be installed.
"An 8GB install is required. For optimal performance, we recommend not installing the play disc," Tweeted Rockstar Support.
Eurogamer's Digital Foundry found installing both discs causes objects to disappear and texture pop-in issues that don't exist when running directly off the disc.