Similar to the Xbox One, a 500GB hard drive has been chosen for internal storage - not exactly surprising bearing in mind that these units currently offer the best value in terms of gigs per £/$. However, the difference is that as with the PlayStation 3 the unit can be replaced with any hard drive of your choosing (and presumably that includes SSDs too). There's a point of deviation with Xbox One here, as the hard drive in the Microsoft console is not user-replaceable. Here, the additional USB ports are used for external devices that boost overall system storage capacity. Bearing in mind that the Blu-ray drive on both systems gives up to 50GB of potential storage for games and that installation is mandatory on both consoles, that hard drive could fill up very, very quickly.
We also have final confirmation on the optical drive utilised - a 6x CAV Blu-ray unit. CAV stands for Constant Angular Velocity, meaning that the disc spins at the same rate no matter where the reading head is. This means that data contained on the outer edge of the disc is read in much faster than that contained on the inner portion. This was much more of a big deal for current-gen games (which mostly run from disc) as opposed to next-gen where hard drive install is mandatory. In order to make the experience as friction-free as possible, games will load the absolute minimum and then "stealth install" the remainder onto the hard drive in the background as you play.
