I ran into the problem with the prohibitory sign on the gray screen on my iMac last week and I wasn’t able to find a solution straight away. After trying zapping the PRAM, doing a safe reboot, starting in target mode, I finally called Apple. I was running 10.6.2.
They wanted to know if I had done a check with the Disc Utility, which I confirmed. The hard disk was OK and I had also repaired the permissions.
They told me to archive and install from the Snow Leopard disk and assured me that the system wouldn’t change the user accounts installed on the machine.
So I proceded to do this. The installation went fine and nothing was changed whatsoever. Software Update alerted me to the fact that I had a combined system update waiting for me (107 MB) after I logged in.
I installed that, and the machine was back to normal again. Talk about an easy way to deal with a system software issue. I expected to be obliged to reinstall applications – maybe even users again, but no. It was smooth sailing all the way
Now if Apple could just come out with a new version of Aperture…