The Xbox One has only been released for a week in select countries, however it is already possible to replace the internal 500gb hard drive with a larger drive. So far a 1TB drive has been tested and confirmed working perfectly. Unlike the Xbox 360 a security sector is not required to be flashed to the hard drive for the Xbox to recognise it, basically meaning if you had a 2TB 2.5” Sata hard drive it can be modified to work in the Xbox One.
To hack / modify a SATA hard drive to work in the Xbox One at the moment you need to be using Linux, a live CD will work fine and reduce the risk if you formatting the wrong hard drive. A person going by the alias of Juvenal has created a script that will automate the process of setting up a new hard drive exactly how the Xbox One wants it, including creating all the correct partitions.
The script simply called Xbox One HDD Creation Tools is open source and can be downloaded from Github: https://github.com/juvenal1/xboxonehdd
When you put the newly modified hard drive in the Xbox One for the first time the console will go the the green Xbox One screen, pause for a second or two then shut down. After this you simply need to power the Xbox One on again, this time the Xbox One should pause at the green screen again, then go to a black screen for even longer. Eventually after several minutes the Xbox will have finished creating the files, setting up the new hard drive and be ready for you to use on the dashboard.
That is basically it, your console’s hard drive is then upgraded. We are now no longer limited to a small 500gb drive, which for a console where games are going on 50gb really is nothing!