Instructions
Move a Gamertag
Turn the Xbox off and attach a storage device (Memory Unit or USB flash drive) to the corresponding port on the Xbox.
Turn the Xbox on. Select the "My Xbox" tab. Select "System Settings." Select "Memory."
Select "All Devices." Select "Gamer Profiles" and select the profile with the Gamertag you want to copy.
Select "Move" and select the appropriate storage device. The gamer profile and all its content (including the Gamertag) will be added to the storage device. After the transfer is complete, remove the storage device.
Connect the storage device to the Xbox you want to copy it to. The Gamertag will be listed alongside any other Gamertags on that Xbox.
Recover a Gamertag
Turn on the Xbox you want to copy the Gamertag to.
Press the "Guide" button on the Xbox controller. Scroll to the "Home" tab.
Select "Recover Gamertag." After the description, select "Recover Gamertag" again.
Use the on-screen keyboard to enter the Windows Live ID information (email and password) associated with the Gamertag (as it was when you created the Gamertag).