Instructions
Format the Drive
Turn on the Xbox 360, and then press the USB drive into the Xbox's USB port.
Select "System Settings" from the dashboard's "My Xbox" menu.
Select "Memory" on the settings screen, and then choose "USB Storage Device" from the list of devices.
Choose "Configure Now" or "Customize," depending on how you want to format the drive. If you select "Configure Now," the entire drive is formatted for the Xbox and all the data is erased. If you select "Customize," you can choose how much space is formatted for the Xbox and avoid erasing data.
Copy the Gamertag
Return to the Xbox dashboard, and select "System Settings" from the "My Xbox" menu.
Press "Memory" on the settings screen, and then select "Memory Unit."
Select "Gamer Profiles."
Choose your gamertag from the listed profiles, and then select "Copy."
Select the USB device from the list of devices.