Important Notice (this was taken from Game Stop site for the 360 version)
Grand Theft Auto V will ship on two discs; Disc 1 will be used for a one-time mandatory install and Disc 2 will be used to play the game. After the install, players will be able to enjoy both Grand Theft Auto V and Grand Theft Auto Online without any need to switch discs.
This initial install will require an Xbox 360 Hard Drive or an external 16GB USB flash drive (sold separately) with at least 8 GB of free space. If using a USB flash drive it must be at least USB 2.0 with a minimum 15mb/s read speed and formatted for Xbox 360 use. A new USB flash drive is recommended to ensure optimum performance.