IMac will not boot after 10.10.2 update

10-11  Source: Network gathering  Views:0 

Hello all, I could really use some help. I downloaded 10.10.2 from the app store on my iMac (previously upgraded to Yosemite through app store), and when the system restarted, I would see the white screen with the apple logo and the progress bar about a third of the way across. then the system would ether restart or hang. Here is what I have done so far.
1. I am unable to boot to recovery mode using 'command R'... nothing happens and it goes back to the apple logo and a progress bar that get to a third of the way and stops.
2. I am able to press 'option' during boot and get 3 options for loading (system, recovery, and a Yosemite install disk i made on a USB). selecting any of these options returns me to the apple logo and the progress bar that gets stuck at a one third complete.
3. I am able to select safe and verbose mode. the machine will sometimes get all the way to "system up time in milliseconds" before it reboots.
4. I am able to boot to single user mode. I have run '/sbin/fsck -fy' until it no longer displays 'modification have been made' and it displays 'volume appears to be ok" (takes 2 runs). from here i have tried both rebooting, and exiting, both result in the white screen with the apple logo and the progress bar getting to a third of the way.
5. i have tried clearing the NVRAM
6. I don't have another mac with firewire or thunderbolt to try targeted disk mode
7. i have had 1 success in getting to the log in screen using verbose mode. during the boot there was a whole screen of the same line repeated many times, it said "BUG in process suhelperdt[167]: over-released legacy external boost assertions (1 total, 1 external, 0 legacy-external). after i logged in it reported that the system has un-expectedly shutdown, and that it would send a report to apple (is this a kernel panic?). i hit  yes and then show the details. then the system hung up, and rebooted. then hung up again during boot with the same "BUG" message above.
I've got good time machine backups, and don't mind doing a complete reload if i can ever get to that option.
Thanks in advance for the help.
I have a 27 inch imac (10,1 late 09) that was doing the same intermittently during 10.10.2.  In a fit of frustration, I updated yesterday to 10.10.3 and now the computer is doing exactly as you describe.  I cannot get past the white screen at all.  I have booted into recovery, verified and fixed the drive even though it it checked out fine.  and attempted a fresh download on a clean partition.  nada...   i can get into safe mode, and I tried installing the SMC fan control, switching off transparency, and lowering my resolution.  none worked. 
I was convinced my hard drive was dying (when I had 10.10.2), and replaced it last week with a SSD.  The SSD ran great for about a week, then had the same problems.  I never had a problem with 10.10.1 or prior.  I have checked my ram, and that does not fix the problem (swapping all four into the front dime slots in sequence). 
This computer has run great since 09, and I have never had any issues until now, but it is now a very expensive paperweight. I cannot do the internet recovery that sets up the original OS the machine shipped with (too old for that feature), and it will not let me download a previous version of OS X from the app store (I have mavericks, lion, etc there). 
I think my apple love affair is officially over.  Looks like i will be searching for a pre-yosimite computer to download a usb version of mavericks or lion, and I sure as **** am not buying another apple product in the near future! I was very close to pulling the trigger on a new macbook, sure glad I did not waste my money -again-.
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