Instructions
Hook your HDD to any Xbox 360 and turn on the console.
Access the Xbox 360 dashboard if necessary by tapping the Guide button on your controller and pressing Y.
Go to "System Settings," located under the far right tab in "My Xbox."
Choose "Memory" from the resulting menu. A prompt will appear asking you to select a storage device.
Use the left thumbstick on your controller to highlight "Hard Drive" from the memory selection screen but do not select it.
Press the Y button to bring up an additional options menu.
Select "Format" from the resulting options menu. Your console will warn you that taking this action deletes all data from the HDD, restoring it to factory settings.
Confirm the format on the resulting prompt. A final prompt will appear requesting your console&'s serial number.
Enter the serial number. Once you enter it, the format process begins.