Instructions
Open the package containing the Xbox 360 HDD unit and remove the HDD.
Locate the protective shell on the Xbox 360 Arcade system and remove it using the tabs on the lateral side. Remove the protective shell and store it somewhere in case you need it in the future.
Attach the HDD to the Xbox 360 system in the place where the protective shell used to reside. Make sure to match up the outlet on the HDD with the port on the Xbox 360 system. Fasten them and snap the other side of the HDD into place.
Turn on the Xbox 360 system and navigate to the system settings tab. Make sure the Xbox 360 system is recognizing your newly installed HDD.