Microsoft has behaved pretty douchily as far as managing the hard disc drive (HDD) sizes it made available for the XBox 360. The 20GB drive included in most models of the system is woefully inadequate, and many have been convinced to give Microsoft even more money to get a larger one. That's not necessary. This video will teach you how to upgrade the XBox 360's hard drive yourself, putting in any large-capacity SATA drive you wish and massively increasing the storage capacity of your XBox 360. Now get to modding!
From the creator:
Things you will need:
T6 (Screw)Driver or Bit
T10 (Screw)Driver or Bit
a PC with SATA Port, Serial Power (or Molex to Serial Power Adaptor) and Xplorer360: http://rapidshare.com/files/125549334/Xplorer360.rar.html
(only use Xplorer360Bevs for WD BEVS Drives)