Photos just will not start. So far since updating I have yet to successfully start Photos. It opens with no window, with all the menu items greyed out except under "Window" where there's a "Photos" option that brings up a blank white splash screen that sits there for about 5 minutes then shows "Welcome" with the picture of the Macbook Pro and a Get Started button. If you click that a little spinning wheel appears underneath forever. The system is otherwise perfectly responsive. I deleted the Photos library that had appeared on my machine thinking it was the problem, but the problem persists. Late 2013 13" rMBP running stock OS X.