I have a 27 " iMac that will not boot all the way.
Looks like it is stuck on trying to do a firmware update at boot up.
it gets to 25% complete and then shuts off
iMac (27-inch Mid 2010), OS X Mountain Lion (10.8.5)
Did you recently attempt to update the firmware? If so and you believe this is a firmware issue you need to take the machine into your local Apple Store or AASP and they can update it for you.
I figured that is what I needed to do, from what I read on otrher forums.
I did try creating the "Firmware Restoration CD" and it doesn't use the CD when doing the firmware install.
I don't believe it is a firmware update. If you are seeing a progress bar during start up you can follow the instructions in http://support.apple.com/kb/TS3148 to resolve it.
A similar problem brought me a solution at:
https://jamfnation.jamfsoftware.com/discussion.html?id=12589
It had me boot into the recovery Partition, shut off the wireless networks, then reboot.
It worked.
The theory has to do with it attempting to authenticate over the wireless and getting stuck there. Shutting off the wireless forces it to use cached credentials (or just stop trying?), so the boot completes through the progress bar.
This thread:
implicates McAfee.
I just had this issue on an iMac I upgraded to Yosemite, thus the late post. We're mostly PC's here. It was too useful not to get the search results into Google.
27" iMac stuck 25% firmware install at boot and shuts off.
