Instructions
Turn on your Xbox 360 and go to the Xbox 360 dashboard.
Go to "System Settings," then select "Memory," then select "Storage Devices."
Move the cursor over "Hard Drive" in the Storage Devices area. Press the "Y" button on your Xbox 360 remote control or controller.
Select "Format" in the Device Options area. The system will prompt you: "This will delete all content on this device. Do you want to continue?" Click "Yes" to continue.