Hello (I think this is the right place to post this...) I have an Early 2011 Macbook Pro 15" with OSX Lion and Windows 7 using bootcamp. I downloaded Mountain Lion from the App Store and made a bootable USB drive. I followed this guide http://www.ifixit.com/Guide/Install...body-Early-2011-Hard-Drive-Replacement/5895/1 and did remember to unplug/plug in the battery cable inside the laptop at the relevant points. I swapped out the original HDD that was in the laptop with a new 640gb Western Digital HDD. When booting with the USB drive, I select the Mountain Lion/USB install, but after some waiting a grey "no entry" sign (circle with diagonal bar through it) appears on the screen and the laptop won't do anything. I turn off/on again and the keyboard backlight comes on and HDD makes a reading noise for a few secs and goes quiet, but the monitor stays blank and nothing more happens. Turning it off/on (yet) again now the HDD makes a noise like it is being read for a few seconds then goes silent, the white light on the front of the laptop comes on but nothing else happens (this time the keyboard backlight in addition to the monitor stays off). I switch out for the old HDD that has OSX Lion on it but the same happens. Note that after initially taking out the OSX HDD whenever I turned the computer on I was lazy and didn't screw the bottom of the laptop back on and instead just pressed it down into place - could this cause a problem when opening the lid of the laptop back up without the screws in place? I have tried turning the laptop on with and without mains power attached and the same happens (the battery indicator shows the battery has full power) Now the laptop is back to how it originally was, with all the screws back in place etc. but will not turn on. I really can't understand how I managed to so completely break it by switching out an HDD (I'm a Computer Science undergrad and have put together my fair share of computers). Luckily there is an Apple store in the city I live in (In England not the US). I have booked a genius appointment and wondered a) Whether there was anything obvious I could do to fix the problem without having to take the laptop to the genius b) What should I tell the genius? I could go for "Tried to install Mountain Lion and now it won't turn on" or I could recall the above story, or something in between the two. As I was only swapping out an HDD, I didn't check to see whether I was breaking the warranty (was I?). In the past I have taken out the fans (using a screwdriver bought on amazon as the fans, but not the HDD, have strange screw heads) from the laptop and cleaned the dust out of them; the last time I did this was a few months ago and the laptop worked fine, but the once perfectly black tops of the screws now have some silver scratch marks on them... will the genius notice/care? Any advice would be awesome. Thanks! tldr: Swapped out HDD from 2011 macbook pro 15" and now it won't turn on, even with the original HDD back in it. What to tell the Apple genius?