You See a Prohibitory Sign When Starting Mountain Lion
When you turn on your Mac, the first thing it does (after the hardware tests) is check for a startup disk that has OS X Mountain Lion (or any version of OS X) on it. If your system doesn’t find such a disk on your internal hard drive, it begins looking elsewhere — on a FireWire, Thunderbolt, Universal Serial Bus (USB) disk or thumb drive, or on DVD.
If you have more than one startup disk attached to your Mac, as many users do, you can choose which one your Mac boots from in the Startup Disk System Preferences pane.
At this point, your Mac usually finds your hard drive, which contains your operating system, and the startup process continues on its merry way with the subtle Apple logo and all the rest. If your Mac can’t find your hard drive (or doesn’t find on it what it needs to boot OS X), you encounter the dreaded prohibitory sign. Think of the prohibitory sign as your Mac’s way of saying, Please provide me a startup disk.
If Apple can figure out a way to put a prohibitory sign on the screen, why the heck can’t the software engineers find a way to put the words I need a startup disk on the screen as well?
If you encounter any of these warning icons, you might need to do some troubleshooting. You can try different options, such as using Disk Tools and First Aid, zapping the parameter RAM (PRAM), and performing a Safe Boot.