Instructions
Power off your Xbox 360 and connect your hard drive to the console&'s hard drive bay, if necessary.
Turn on the Xbox 360 and go to the dashboard by pressing the Guide button on the controller, then pressing "Y."
Go to "My Xbox" from the dashboard menu, then press the "RB" button until you&'ve found "System Settings."
Press "A" to open "System Settings."
Choose "Memory" from the menu. A new prompt will appear asking you to choose your desired storage device. But don&'t press "A" immediately on this screen.
Highlight "Hard drive" from the menu, then press "Y." This brings up an alternate options screen relating to the hard drive.
Choose "Format," then confirm that you would like to erase all data on the hard drive on the following prompt by selecting "Yes."
Enter your Xbox 360&'s serial number, if applicable. The hard drive restoration process will then begin.