When I tried to start up my Mac one morning It wouldn't boot, it simply gave a rhythmic groan (dundun-dun). I've tried a load of disk utilities that I had on a disc (Drive Genius 2) and I think it looks like a bad sector in the System file, of all places. I read another thread in a different forum and he told a guy to hold down command-v while booting to go into verbose to find out what the problem is. I did. In rhythm with the hard drive, the command "Disk 0s2 I/O Error" but I know that it is connected because I ran all of the disk utilities fine. One of the utilities that was, thankfully, on my tools disk (I highly recommend creating one) was Carbon Copy Cloner so I have copied everything to a USB drive which I am running on now. I have also run OS X Disk Utility once I had booted off the USB HD and I tried to repair Macintosh HD but it came up fine (I guess that was from the huge amount of disk utilities I had used!) but when I tried to repair permissions, there was a huge list of things that it repaired. Please help, you only realize how much you rely on your computer when you can't use it.