When you turn on your Mac, you a see a flashing question mark. It may continue to load up normally.
If you are able to login after seeing the question mark:
- Click the Apple icon in the top left of the screen.
- Click System Preferences.
- Click the Startup Disk option.
- Select the main hard drive which you have Mac OS X installed on.
If the screen stays on the question mark:
- Shut down the Mac (hold down the power button to force it to turn off if you need to).
- Turn it on and hold down the alt key.
- You should have a menu asking you to choose a hard disk. Click on you main hard drive.