Instructions
Connect the hard drive to the Xbox 360, if necessary. You can reformat it from any console.
Power on the console.
Press the Xbox Guide button on your controller to bring up the Guide menu.
Scroll right to the "Settings" tab and choose "System Settings." This will redirect you to a list of system settings options.
Choose "Memory." You will then be prompted to choose a storage device.
Highlight the hard drive and press the "Y" button to view additional hard-drive options.
Choose "Format" and confirm the action on the following prompt.
Enter your console's serial number to begin the process.