OS X Yosemite won't get to login screen

[ME1312]

My IMac boots normally until like 40% then goes to a black screen with a buffering img. Help?

 

[Taz Mangus]

My IMac boots normally until like 40% then goes to a black screen with a buffering img. Help?

What do you mean by "40%"? Does it ever boot to the login screen? I would suggest creating a second partition and doing a clean install on the second partition. You might need to boot to the recovery partition using Cmd-R. If you find it booting fine with the clean install then I suggest a clean install on the primary partition. If it does the same thing again, then I suggest a call to AppleCare. You do get free AppleCare phone support for Yosemite. Could be a hard drive issue.