I realize that this is a very late response, but perhaps it will help others who search the "Answers Index" area here.
'''Have you tried a(n) SMC reset? This can "cure" a lot of issues.'''
* '''Reset SMC of an iMac, Mac Pro, Mac Mini:'''
* Shut down the computer.
* Unplug the computer's power cord.
* Wait fifteen - twenty seconds.
* Attach the computer's power cord.
* Wait five seconds, then press the power button to turn on the computer.
'''OR'''
* Shut down your Mac
* Disconnect the power cord
* Wait fifteen - twenty seconds.
* Press and hold the Mac’s power button for 5 seconds
* Release the button
* Reattach the power cables and boot the Mac as usual