When first starting with my OS (Snow Leopard) disk all I got was a whitish or gray screen, not even the mouse. So I started freaking out a bit. I couldn't get the key board to stay on so I kept pushing the keyboard power button while holding the option button and then the mouse arrow came up on the screen. After that the apple sign and then the install procedure started, so I was able to breathe again. So about five in the morning I was able to let it do it's thing and get some sleep. The next day I was able to restore from the external hard drive using Time Machine and so far my IMac 20" 7,1 #2133 is running super fast again probably better than when I got it brand new " if that's possible ". With your instructions on I fixit.com and your videos on YouTube it was pretty darn easy, even for someone like me who has never worked on a computer. The most I've done on a computer I installed a new program (Snow Leopard). Thank you, Tom D.