well actually every x box has a 10 gig HDD
but each x box use 2 gigs of cach for certain games, plus the xbox OS an HDD partition.

here are the official specs on the Harddrive contains

- Windows 2000 Core OS
(Custom designed by MS)
- DirectX 8.0a (Drivers)
- Part 1 of the OS is on the hard disk < 1 MB
- Part 2 of the OS can be DVD disc < 500 K
- (Part 1 includes the core OS, DirectX, DVD playback, some drivers and 3D user interface)
- (Part 2 includes things like libraries, other drivers and other features needed by the developer)
- OS takes less then 3 MB in RAM
- The OS has a 3D user interface when no games are inserted to play music CDs, run DVD movies etc.
- The games run in ring 0, known as kernel mode which is the fastest mode possible
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