If a flashing question mark appears for just a few seconds
If your computer starts up normally after showing a flashing question mark or flashing globe for just a few moments, you may need to reselect the startup disk in Startup Disk preferences. You can select a startup disk in System Preferences to resolve this.
- Choose System Preferences from the Apple menu
- Click the Startup Disk icon in the System Preferences window
- Click the icon of the disk you normally use to start up your computer (by default this is named "Macintosh HD") so that it is highlighted.
- Close the System Preferences window.
If your Mac doesn't get past the flashing question mark
If your Mac starts up to a gray screen (with no flashing question mark) or a flashing question mark or flashing globe, give the computer a few more moments to locate its system software. If you normally start your computer from an external drive (Thunderbolt, FireWire or USB) make sure the drive is connected and powered on.
If your computer still does not start up, try the next set of steps to help your computer find its system software.
- Turn off your Mac by pressing and holding its power button for a few seconds.
- Press the power button once to turn your Mac back on. Then, hold down the Option key on your keyboard until the Startup Manager appears.
- Select your startup disk from the list of drives that appear on screen.
- If your computer finishes starting up normally, choose System Preferences from the Apple menu. Then, click the Startup Disk icon in the System Preferences window.
- Select your normal startup volume (such as Macintosh HD) from the list of drives that appear in the Startup Disk window.
If your installation of OS X needs repair
If you don't see your startup disk displayed in Startup Manager, or if you see a prohibitory symbol (⊘) appear, your startup drive may need directory repair, or OS X may need to be reinstalled.
- If you only see a disk named "Recovery HD" in Startup Manager, select it instead. You can also start from OS X Recovery by holding down Command and R on the keyboard at startup.
- From the Utilities menu, choose Disk Utility.
- In the Disk Utility window that appears, select your startup disk (usually named "Macintosh HD") from the left side of the Disk Utility window.
- Click the First Aid tab.
- Click Repair Disk to verify and repair any issues with your OS X startup disk.
- After repairing the disk, try to start up normally.
- If none of these steps resolve the issue, start from OS X Recovery and reinstall OS X.
If Disk Utility finds issues it can't repair
If Disk Utility finds issues on your startup disk that it cannot repair, you may need to back up as much of your data as possible to another drive (or use Time Machine to back up). Then erase your normal startup disk and reinstall OS X. Alternatively, install OS X onto an external drive and use Migration Assistant to move your important data to the external drive. You can then erase your normal startup disk and resinstall OS X. Use migration assistant a second time to migrate your data from the external drive back to your normal startup disk.
Important: Erasing a disk deletes everything on that disk (including things on your desktop). Make sure you have backed up your important data stored on a disk before erasing it.