Same problem on Macbook Pro (early 2011). Going through various start-up procedures with an Apple adviser - resetting parameter ram etc -- none of which remedies were successful -- we started up in Safe Mode (press Shift after the start-up tone but while the screen is still black and hold until the Apple symbol and spinning gear appear). You will have to sign in using your password after this. After start up is complete, select restart from the pull-down menu and you may find the problem is cleared, it was on mine.
After this procedure, i find that a file named 'mach_kernel' -- which is usually invisible -- has appeared at the top level of the hard drive. This is apparently an important file and should not be moved or deleted. Quite how you restore it to its rightful--unseen--place I've yet to discover. I found one reference to it on another thread which gave a command-line instruction to use via Terminal. Didn't work though -- "command not recognised". Anyone gets to the bottom of that one I'd be pleased to hear...
Incidentally I've discoved via a highly learned thread that no authentic Mac geek uses Software Update (the very idea!) but those in the know download the combo update and install direct thereby -- apparently -- avoiding all these horrors. Who knew?
