Boot hangs on grey screen (even when booting from USB drive with fresh OS X install)
Solved: Booted to Single-User mode and disabled Radeon GPU.

Details of investigation:

Initially I suspected hard-drive corruption and went about trying to remedy that. Unsuccessfully, I tried the following, with each continuing to hang as described above:


Safe boot
Boot into recovery (including Internet Recovery)
Boot from install media on USB drive
Boot from OS X installation on USB drive
Clear NVRAM
Reset SMC


I also ran the Apple Hardware Test many times without it finding any issues.

Verbose safe boot (Cmd+Shift+V) output everything that I'd expect to see but would then hang as described above.

After coming across more posts online of GPU related problems I revisited this as the cause:


https://discussions.apple.com/message/22751155#22751155
https://discussions.apple.com/thread/4766577?start=210&amp;tstart=0


Attempting to boot Ubuntu from a USB flash drive, I could only get as far as Grub. When trying to boot Ubuntu Desktop or run the graphicstest in Grub the system would hang.

At this point, running Apple Hardware Test hung right before the end of the standard test, possibly [guessing] when doing a video test.

Based on the advice in the Apple Discussions posts above I did the following:


Boot into Single-User mode
Execute the following commands:



    /sbin/fsck -fy /
    /sbin/mount -uw /
    mkdir /Disabled_System_Library_Extensions
    cd /Disabled_System_Library_Extensions
    mv /System/Library/Extensions/ATI* .
    mv /System/Library/Extensions/AMD* .
    touch /System/Library/Extensions
    exit


This time the machine booted all the way through. However, graphics are extremely slow, even just transitions when minimizing windows. I will be taking my MBP to Apple to demand a replacement as the large number of reports of others facing similar issues makes it look like a recurrence of a similar GPU-related failure that resulted in them doing a recall.