While onsite at a client in Chicago last month, I updated a 27″ iMac (late 2012) from OS X 10.9.3 to 10.9.4. After rebooting the Mac would just hang at the gray screen. I was able to boot into Single User Mode and run fsck, which said everything was fine, but upon reboot it still hung at gray screen.

I tried all the basics including: Clearing the NVRAM/PRAM, resetting the SMC, disconnecting all peripherals, and selecting the startup disk after rebooting hold down the option key.

I found it would boot fine from my external USB service drive, and I even ran Disk Utility on the iMac’s boot drive. Still the same hang at gray screen when attempting to boot from it.

Then I tried something I rarely have need to do… I booted the Mac into Verbose Mode boot (holding down the command+v) to see if I could determine what was causing the hang. The last thing listed was “pci pause: sdxc”.

I googled this, and found a recent thread on the Apple Discussion Forums detailing the same problem with the same model iMac. Apparently the Mac was trying to run fsck during the boot, was getting hung up. The workaround posted was to boot into Single User Mode, rename the fsck Unix executable, then replace it with a blank fsck file.

I tried this, and sure it enough it allowed the iMac to boot properly! Afterwards I booted back into Single User Mode and put fsck back to the way it is supposed to be, then tested. It continued to work fine.

For reference, this is the discussion thread I used…
https://discussions.apple.com/thread/6434619

And these are the steps I personally did to fix the problem…
___1. Boot into Single User Mode
___2. /sbin/mount -uw /
___3. mv /sbin/fsck /sbin/fsck.off
___4. cp /usr/bin/true /sbin/fsck
___5. Rebooted Mac, and verified it booted properly
___6. Boot into Single User Mode
___7. /sbin/mount -uw /
___8. rm /sbin/fsck
___9. mv /sbin/fsck.off /sbin/fsck
___10. Rebooted Mac, and verified it continued to boot properly