My iMac is being very odd. It won't boot using the internal hard drive. It chimes, starts the Yosemite progress bar and then changes and looks as if it is about to show me the login screen but then it just hangs at a grey screen. After about 30 seconds it just shuts down.
My first thought was that it is just a software issue or hard drive failure so I attempted to boot up on my USB stick with Yosemite installer on it and then another one with TechTools Pro on it, they all fail at the same point as above.
I booted up and did a Apple Hardware Test, the initial test came back OK. The extended test came back OK also.
Just for the hell of it, I have put a different hard drive in it but still can't boot to USB stick to install OS X. Grr.
Any other suggestions would be gladly and gratefully received. Thank you :)