I have been trying to understand this issue for a couple weeks now. I put a new copy of Yosemite on an old (wiped) hard drive and it loads and operates normally on a 2.4GHz 13" 2010. But then you'll let it sit, or restart it after something happens where it needs to be restarted and it just hangs on the Apple boot screen. At first I thought it was depending on the wifi as I noticed one time when I turned the wifi on and then turned the computer off, tried to open and log in, but it wouldn't get past the Apple logo and about 1/4 - 1/3 of the loading bar. Every time I would turn off and on it would do this, so I would just reformat the hard drive on a different computer then reinstall yosemite again and it'd work for awhile and eventually does the same thing, but maybe a different circumstance. I'm starting to think it's the hard drive, but not really sure on how to diagnose this problem of after being shut down, not being able to restart again. Any tips or pointers would be greatly appreciated!