External CD drive is connected to Mac Air. Trying to book Mac Air to Install DVD. At reboot I hold down C and then Apple logo appears. I've done this before on my Mac Mini and Installation Screen pops up after a minute. But with the Mac Air it just hangs and after 3 minutes i get a message "You need to restart your computer. Hold down the power button until it turns off, then press the power button again". I did that and still same thing happens over again. Any suggestions?