Instructions
Power off the Xbox 360 console. Removing or installing a HDD while the console is on can damage the HDD or Xbox 360 or create errors when reading the HDD.
Remove the old HDD if neccessary by pressing and holding the gray eject button, then pulling the HDD off of the console.
Begin changing HDDs by aligning the new HDD with the now-empty HDD bay. Notice that the skinnier side of the HDD has a plug-in. Make sure that this plug-in aligns with the HDD port in the HDD bay.
Push the larger, fatter side of the HDD down first. If you attempt to push the skinnier side in first, the HDD will not connect correctly. Push the larger side in as far as it will go.
Push the skinnier side in until it locks into place. You will know when it locks in when it makes a clicking noise.