MacBook Pro 2011 shows grey screen during boot

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&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.

If you have your original discs load up the Applications Install disc and run the Apple Hardware Test from there. Look for memory faults. I doubt very much you have a firmware "bug", memory failure is much more likely and would explain the unusual hardware behaviour you're experiencing.

I had the same model which I initially bought around 5 years ago, this had the original OS installed (10.5.6). Immediately after purchase I used the install discs to wipe the machine totally clear of the previous owner's setup, no problems encountered during installation.

Everything seemed to generally work ok (one or two occasional random crashes) until I decided to wipe it clean following the retail release of 10.6. After trying (and failing) several times to install 10.6 I decided to substitute the factory 1GB RAM stick with a 512MB stick from another laptop. This time 10.6 installed perfectly, first go. Faulty memory explained the random crashes as well as the installation issues with 10.6. Incidentally, I tried running the machine after installing 10.6 with its original 1GB RAM. It would sometimes boot but was completely unpredictable - single user mode was always accessible though, like your setup.