Are you on Mac Maverick? You need to reset your NVRAM.
1. Shut down the computer.
2. Wait for a while. Beboot.
3. Turn on Mac without opening any programmes.
4. Press and hold the Command-Option-P-R keys.
You must press this key combination before the
gray screen appears.
5. Hold the keys down until the computer restarts and
you hear the startup sound for the second time.
6. Release the keys.
Are you on Mac Maverick? You need to reset your NVRAM.
1. Shut down the computer.
2. Wait for a while. Beboot.
3. Turn on Mac without opening any programmes.
4. Press and hold the Command-Option-P-R keys.
You must press this key combination before the
gray screen appears.
5. Hold the keys down until the computer restarts and
you hear the startup sound for the second time.
6. Release the keys.