Upgraded to Yosemite from Mavericks on a top end iMac with 3TB fusion drive and when I switch it on and open safari it's fine for about 5 minutes then I get the spinning wheel and I can't open anything else. Mouse moves but nothing opens I have to power off at the back. Spotlight search no good either. Typed in terminal and it just gave me a Wikipedia reference not an option to open the app. What should I do? Try installing again from App Store. Have time machine backups from last week but not sure if you can restore full OS from that? It's fine on my MacBook pro but I don't have as much software on that. Need it for Working on tomorrow so need to sort tonight.