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Some keyboards have a light that flashes briefly at startup, indicating that the keyboard is recognized and ready for use. You might need to wait a few seconds before pressing the keys, to give your Mac more time to recognize the keyboard as it starts up. Then press and hold the keys as your Mac starts up. Then press the power button to turn on your Mac.
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Option-Command-P-R: Reset NVRAM or PRAM.If your Mac is using a firmware password, you're prompted to enter the password. Option (⌥) or Alt: Start up to Startup Manager, which allows you to choose other available startup disks or volumes.macOS Recovery installs different versions of macOS, depending on the key combination you use. If your Mac is using a firmware password, you're prompted to enter the password. Or use Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the internet. Command (⌘)-R: Start up from the built-in macOS Recovery system.Keep holding until the described behavior occurs. To use any of these key combinations, press and hold the keys immediately after pressing the power button to turn on your Mac, or after your Mac begins to restart.