I upgraded to Yosemite about a week ago on my work computer. All went well, just did an upgrade from Mavericks on my 27" late 2013 iMac. Before I did the upgrade I did back-up everything, ran disk permissions and verify disk, all was good. When I go home for the evening I put the computer to sleep. A few days after the upgrade I came into work and the screen was on with only a circle with a line through it. Did a reboot, checked the disk again and all seemed ok. Another day went by and it happened again. Now it is happening daily, normally it would start up just fine but today it would not. I was able to get it to boot in safe mode and all checks out fine. What is causing the machine to have to reboot overnight? I don't have auto updates on. Why is it having problems finding the start-up disk? I've done a few searches and have not found anyone with a current system having this issue. My HD is the 3TB Fusion Drive, did not have any issues until I upgraded which leads me to believe it's a software issue and not hardware but could use some help. Thanks.