I tried to upgrade to Mac OSX 10.6 today (it was running 10.4) but it failed. When I tried to restart and try again, it brought up a pop up window that said it could only install if existing info was erased. It gave the option to cancel and restart using old system 10.4. Since then, the computer turns off soon after the gray screen appears. I tried to reboot holding shift S and seeing if it will reboot, but it gives me a "volume not found" message.