Instructions
Power off your Xbox and turn it on its side, so the hard drive is visible. The hard drive is rectangular in shape and has "HDD" written on it.
To unlock, locate the circular release button at the end of the hard drive. Press this button in to unlock it and simultaneously pull the drive off of the Xbox to remove it.
To lock, fit the hard drive back over the side of the Xbox and press it down until the button locks it in place.