UPDATE 3: Digital Foundry has investigated the matter further and discovered a possible solution to the Grand Theft Auto 5 Xbox 360 "play" disc installation issue.
The video, below, shows footage of the game played with the "install disc" installed to the Xbox 360 hard drive, as is mandatory, and the "play disc" installed to a USB flash drive. Doing so appears to eradicate the performance issues found when both discs are installed to the Xbox 360 hard drive.
UPDATE 2: Digital Foundry has created a new video, below, that shows why it's best Xbox 360 gamers follow Rockstar's advice and not install the Grand Theft Auto 5 "play" disc.
UPDATE 1: Digital Foundry, which has access to the Xbox 360 version of Grand Theft Auto 5, has offered an explanation for Rockstar's recommendation against installing the "play" disc.
"Rockstar recommends that you shouldn't install the GTA 5 play disc on Xbox 360 and it's sound advice," Digital Foundry chief Richard Leadbetter said.
"When you first get the game, the first order of business is installing the mandatory content from disc one - this accounts for 7.7GB of data to install on your hard drive. Now, there's absolutely nothing stopping you installing disc two - the 'play' disc - onto the hard drive too, using the Xbox 360's dashboard to do so. It takes up an additional 7.8GB of storage. However, doing so impacts the game's ability to stream texture data, resulting in regular pop-in of in-game objects and textures.
"So what's the story here? Well, optimal streaming is achieved by making use of all the available bandwidth in the system. Why stream just from the hard drive when you can run in data simultaneously from both the disc and the HDD? Based on what we're seeing on the Xbox 360 version, perhaps running both DVD and HDD assets from just the one source slows down access times, impacting streaming performance. It's not game-breaking stuff, but it does take you out of the moment when it does manifest and for that reason we can't recommend installing the play disc.
"GTA 5 isn't available for digital delivery on Xbox Live, but we are curious what the impact - if any - will be on the PS3 version, as that is available as a PSN download. The Digital Foundry Face-Off - due tomorrow morning at 11am - is based on comparison of the two retail disc versions of the game."
ORIGINAL STORY: Rockstar has warned Xbox 360 gamers not to install the Grand Theft Auto 5 "play" disc.
On Xbox 360 the openworld game comes on two discs: one mandatory "install" disc and one "play" disc, both around 8GB in size. It's a similar situation to Forza 4, which also shipped on two discs on Xbox 360.
Prospective GTA 5 owners - some of which already have the game as a result of early shipping from Amazon.co.uk - might have wanted to install both discs, but Rockstar recommends against it.
"For optimal performance, we recommend not installing that disc," tweeted Rockstar Support.
Then, "An 8GB install is required. For optimal performance, we recommend not installing the play disc."
Rockstar has promised more information soon. For now, the upshot is, with GTA 5 on Xbox 360, you have to install disc one but run disc two from the drive.