I downloaded Mountain Lion and installed it on my main laptop (2011 MacBook Air 11") yesterday. Today I tried installing it on my backup machine (2009 MacBook Air 13") with very different results. Partway through the installation, I got a warning message that the hard drive was damaged (although it had been working entirely normally until I tried to install Mountain Lion). I used Disk Utilities to repair and tried again. The install, which showed 37 minutes to complete, took several hours to get down to zero and then started counting into minus figures (-6 minutes, etc.) with the blue bar complete but the installation not finalising. After a couple more tries, I attempted to reboot from the Leopard and Snow Leopard discs. The former didn't work at all, and the later indicated that it couldn't use the hard drive as a startup disk. Once again, I used Disk Utilities which said it had been properly repaired and was now error-free, but when I tried again I got the same results. What now? Without booting from a DVD, it just gets stuck in an endless loop of trying to install Mountain Lion, but the cycle never completes. Could Mountain Lion in some way have destroyed the hard drive? (Makes no sense, but it seems one hell of a coincidence.) What should I do next? Should I try erasing the hard drive altogether and then rebooting from the Leopard or Snow Leopard disk, then updating to Lion?