At my school, a friend of mine brought in his iMac so we could use it for music production. We set it up and got it working but in a few days when we went to turn it on it only showed the apple logo with the spinning wheel and nothing happened. I've tried starting it in safe mode, single-user mode, verbose mode, and I've even tried resetting the PRAM. When trying to boot in single-user mode it says a disk0s2: I/0 error. What does that mean and is there anyway we can fix it? PS: It's also running snow leopard if that helps.