Eu tive esse problema também com um 820i e um iMac executando 10.11.5 (El Capitan). Eu consegui trabalhar novamente fazendo um SMC Reset & Redefinir PRAM.
Para fazer isso, eu segui estas instruções :
SMC RESET
Shut down the computer. Unplug the computer's power cord from both the wall and computer. Wait 15 seconds. Attach the computers power cable. Wait another 5 seconds and press the power button to turn on the computer.
PRAM RESET
Shut down the computer. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4. Turn on the computer. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears. Hold the keys down until the computer restarts and you hear the startup sound for the second time. Release the keys.