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 🙂
Best Answer
You should try to boot your OS in verbose mode or safe mode to see whether you can fix it or not.
https://support.apple.com/en-us/HT201262
https://support.apple.com/en-us/HT201573
If you really need to reinstall Mac OS X Yosemite, I'd suggest you run this script from any Mac, you need to have Yosemite downloaded in /Applications.
Credit goes to the person who wrote that script for Maverick
You can then restore the ISO using Disk Utility on a 8Gb USB Stick and it will be bootable.