Instructions
Turn on your Xbox 360 and connect your hard drive to the console if necessary.
Open the dashboard by pressing the Xbox Guide button on your controller, pressing "Y," then selecting "Yes." Your Xbox 360 may automatically boot to the dashboard, depending on your settings.
Open your "System Settings" from the "My Xbox" section of the dashboard. This opens a new menu.
Select "Memory" from the "System Settings" menu list. This opens a dialogue window where you can select a storage device to view its content.
Scroll to your hard drive then press "Y" to open a new "Device Options" window.
Choose "Format" from the "Device Options" menu list. Your Xbox 360 will open a prompt informing you that this will delete all data from your hard drive.
Choose "Yes." Your Xbox 360 will then begin formatting and deleting everything off the hard drive.