Instructions
Plug the USB hard drive into the USB slot located on the front of the Xbox 360.
Access the "System Settings" menu located in the "My Xbox" submenu in the Xbox Dashboard.
Open the "Memory" menu and choose "USB storage device" on the resulting storage device screen.
Choose the "Configure Now" option on the screen. This will delete all previous data on the USB hard drive and configure it for use with the Xbox 360. The USB hard drive is now configured and available for use as a hard drive for your Xbox 360.