Follow these steps on older computers that have neither Apple silicon nor the Apple T2 Security Chip.
Laptop computer
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Shut down your Mac.
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On your built-in keyboard, press and hold all of these keys:
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Shift
on the left side of your keyboard -
Control
on the left side of your keyboard -
Option (Alt)
on the left side of your keyboard
-
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While holding all three keys, press and hold the power button as well.
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Keep holding all four keys for 10 seconds.
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Release all keys, then press the power button to turn on your Mac.
If your Mac isn't a laptop computer, follow these steps:
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Shut down your Mac, then unplug the power cord.
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Wait 15 seconds, then plug the power cord back in.
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Wait 5 seconds, then press the power button to turn on your Mac.