About Startup Disks and Booting
Although you usually see a stylish Apple logo when you turn on your computer, once in a blue moon, you might not. You might instead see a solid blue screen, a solid gray screen, a solid black screen, or something else entirely, as described in the next section.
The point is that your Mac isn’t starting up as it should. When this happens, it usually indicates that something bad has happened to your Mac. Sometimes, it’s a hardware component that has bitten the dust; at other times, OS X itself has somehow been damaged.
Rest assured that these occurrences are rather uncommon — many Macs and Mac users go an entire lifetime without seeing one. If you ever have a Mac that won’t boot, don’t despair. Before you declare your Mac terminally ill, try out the advice in this chapter.
Finding or creating a startup disk