So I did the upgrade to Yosemite for my aunt's MBP and it installed fine. Then I did her iMac and it's now stuck on white screen after it restart, for a long time. I shut it off and turn it back on and see the Apple icon and the status bar moving about one third of the way and the screen went to white. I don't know what to do now. I told her I'll go back and check it again tomorrow. Just let it come back. Hopefully. Any suggestion? Thank you.