Just two days after installing the newest Apple OS X 10.8 Mountain Lion in my 27” 2TB iMac the computer crashed. I was cut off from over 300 GB of valuable data, much of it not backed up. Two hours on the telephone with AppleCare did not solve the problem. I was referred to an Apple Store where the computer was expertly repaired in a day without loss of data.
This has been a learning experience. I have meanwhile learned how to use Time Machine (which I did not previously understand) and am now fully backed up in both computers that I use.