Instructions
Purchase an Xbox 360-specific hard drive. Unlike other game consoles, the Xbox 360 can only use hard drives specifically formatted for the console. You can purchase Xbox 360 hard drives at gaming stores and electronics retailers.
Disconnect your Xbox 360 from your TV and from its power source. It is very important that there is no power flowing to the Xbox 360 while you are installing the new hard drive as it has not been "recognized" by the console yet and can cause errors.
Press the latch release button on the top of the Xbox 360 arcade and then pull the plastic cover off. The hard drive bay is now exposed.
Place the round end of the hard drive against the back of the hard drive bay, then gently lower the hard drive into place, making sure all the metal prongs match up with their respective connector jacks.
Release the latch button. The hard drive should now be locked into place. Then reconnect the A/V cables and the power and start up the Xbox 360. The hard drive will be recognized and formatted immediately.