I Found Thisfound this according to my Problemproblem in this thread :
Boot hangs on grey screen (even when booting from USB drive with fresh OS X install)
My MacBook Pro 15" Early 2011 with AMD Radeon HD 6750M exhibited display corruption and associated system crashes/resets over a period of two weeks before it entirely failed to boot. The boot would progress through the grey screen with the Apple logo and spinner, but just when it seems it should have switched to the login screen the Apple logo and spinner would disappear and hang on a blank grey screen.
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:
Boot into recovery (including Internet Recovery)
Boot from install media on USB drive
Boot from OS X installation on USB drive
Reset SMC I
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 at Apple's discussion forums of GPU related problems I revisited this as the cause:
https://discussions.apple.com/message/22751155#227511552011 MacBook Pro and Discrete Graphics Card or https://discussions.apple.com/thread/4766577?start=210&tstart=02011 MacBook Pro and Discrete Graphics Card Attempting
Attempting to boot Ubuntu from a USB flash drive, I could only get as far as GrubGRUB. When trying to boot Ubuntu Desktop or run the graphicstest in GrubGRUB 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.
But when I use the "mv" command the files won't get moved (neither deleted) and it showshows me :
Sandbox deny(01) file-write-unlinked ..... anyunlinked…
Any Solution ?