The prohibitory sign appeared when I started up my macbook pro. I read through the various options listed on this forum and none of them fixed the problem. So I assumed that it was my hard drive or a corrupt operating system. I reinstalled OS X. Still had the problem. Finally, I swapped out my old hard drive with a new hard drive. Still had the problem!! When I tried my old hard drive in my spouse's macbook pro (same model) it booted up just fine! When I but his hard drive (which was working fine) into my macbook pro, I got the prohibitory sign. So I'm assuming it is not my hard drive but some other hardware issue in my macbook pro. Does anyone have any insights into what that hardware issue might be? Thanks!!