Some weeks ago my late 2007 MacBook Pro 15” started having issues during boot – it presented gray Apple logo with a bar graph underneath which went somewhere near 1/4th the way two times and then the computer would just turn off without any warning. I immediately checked various forums and tried all suggestions, but nothing worked. Luckily I had my Tech Tool Pro 5 DVD ready so I booted with it and performed all bunch of tests, even waited an hour for HD bad sectors search, however no problem whatsoever was reported. I also performed volume rebuild, restarted and voila – OS X booted (still preceded with gray bar graph though). Once there, I ran Disk Utility to repair permissions and it didn’t report any problem.