Instructions
Detach the smaller hard drive from your Xbox 360 by pressing and holding the "Eject" button on the hard drive.
Pull the smaller hard drive off the Xbox 360 while holding the "Eject" button.
Line up your larger Xbox 360 hard drive in the now empty hard drive bay. Ensure the fatter, round side positions toward the audio-video ports on the console and the more slender, square side (the side with the "Eject" button), lines up toward the disc tray on the front of the console.
Push the fatter side down first. It won&'t click into place, but push it as far down as you can.
Push the slender side down until the hard drive locks into place, indicated by a clicking noise.