Instructions
Locate your console's old hard drive, positioned on the thin side of the Xbox 360, closest to the disc eject button. Notice that one side of the hard drive is rounded and protruding from the console, while the opposite end of the hard drive (the one facing towards the front of the console) is square, sitting flush with the rest of the Xbox 360. Be sure the console is off.
Press and hold down the hard drive's gray button. This is the eject button.
Grasp the rounded, protruding end of the hard drive and pull on it until it detaches from the Xbox 360.
Align your 120GB hard drive identically to how your old hard drive sat in the empty hard drive bay.
Push down on the rounded, protruding end of the 120GB hard drive until it won't push down anymore.
Press down on the thin side until it sits flush with the Xbox 360, locking into place.