''operating system not found'' problems. I had this come up and I unscrewed the hard drive back cover and found that it had dislodged and the screws were missing. I previously had a problem fixed by ''experts'' who must have forgotten to replace the screws. Computer is working fine now. So all the suggestions to insert back up disk, reboot, recover disks etc and 'your hard drive is dead' were incorrect. It was a mechanical problem. Hope this helps.