Macintosh Startup Key Combinations. This document is loosely based on Charles Poynton's startup key combinations document which has since disappeared from his web site. A Mac does several checks when it is first powered on; one of the first things it does is scan the keyboard bus (ADB) for any signals. Startup key combinations are invoked immediately following the startup chime, and before the grey Apple logo appears in the middle of the screen, when you power-on your Mac. The object of these key combinations is to perform different functions, many of which are useful in the maintenance of Macs. Here is a handy listing of the most important startup and shutdown key combinations and related techniques, applicable to Macs running macOS Sierra and later. Instead of giving these by the keys used, I have ordered them by function, as that is how you will want to look them up. The Mac’s most useful startup keyboard combos. First, here are the available startup key combinations. You can also see a list over at Apple’s support pages. To use them, you should press the.

A sequence of keyboard key combinations are really useful when doing troubleshooting on your Mac computers (Macbook Air, Mini, iMac or Macbook pro's),

Most Important MAC Startup Key Combinations — Tech Support

You need to press and hold these key combinations immediately after you press the power, you can use them even after your Mac reboots. Make sure you keep holding the keys until you see the described behavior.

• C → boot from CD - gone are the CD-Rom days though :)
• T → To boot in target disk mode.
• D → To boot in Apple Diagnostics utility.
• N →To boot from NetBoot server
• Command (⌘) + R → To start macOS Recovery System.
• Option (⌥) + Command (⌘) + R → To start macOS Recovery mode over the Internet.
• Shift + Option (⌥) + Command + R → Start macOS Recovery mode over the Internet.
• Option (⌥) → This will display the Startup Manager
• Alt → This will display the Startup Manager. You can choose from various options such as startup disk or volume.
• Option (⌥) + Command (⌘) + P + R → Reset NVRAM or PRAM.
• Shift (⇧) → To Startup in safe mode.
• Command (⌘) + S → To boot in single-user mode.
• Command (⌘) + V → To boot in verbose mode.
• Eject (⏏) → To remove/eject media, such as an optical disc.
• F12 → To remove/eject media, such as an optical disc.
• Mouse Button or Trackpad Button → To remove/eject media, such as an optical disc.

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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. Keep holding until the described behavior occurs.

• Command (⌘)-R: Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the Internet. macOS Recovery installs different versions of macOS, depending on the key combination you use while starting up. 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. If your Mac is using a firmware password, you're prompted to enter the password.
  
• Option-Command-P-R:Reset NVRAM or PRAM. If your Mac is using a firmware password, it ignores this key combination or starts up from macOS Recovery.
• Shift (⇧): Start up in safe mode. Disabled when using a firmware password.
• D: Start up to the Apple Diagnostics utility. Or use Option-Dto start up to this utility over the Internet. Disabled when using a firmware password.
• N: Start up from a NetBoot server, if your Mac supports network startup volumes. To use the default boot image on the server, hold down Option-N instead. Disabled when using a firmware password.
• Command-S: Start up in single-user mode. Disabled in macOS Mojave or later, or when using a firmware password.
• T: Start up in target disk mode. Disabled when using a firmware password.
• Command-V: Start up in verbose mode. Disabled when using a firmware password.
• Eject (⏏) or F12 or mouse button or trackpad button: Eject removable media, such as an optical disc. Disabled when using a firmware password.

If a key combination doesn't work

If a key combination doesn't work at startup, one of these these solutions might help:

• Be sure to press and hold all keys in the combination together, not one at a time.
• Shut down your Mac. Then press the power button to turn on your Mac. Then press and hold the keys as your Mac starts up.
• Wait a few seconds before pressing the keys, to give your Mac more time to recognize the keyboard as it starts up. Some keyboards have a light that flashes briefly at startup, indicating that the keyboard is recognized and ready for use.
• If you're using a wireless keyboard, plug it into your Mac, if possible. Or use your built-in keyboard or a wired keyboard. If you're using a keyboard made for a PC, such as a keyboard with a Windows logo, try a keyboard made for Mac.
• If you're using Boot Camp to start up from Microsoft Windows, set Startup Disk preferences to start up from macOS instead. Then shut down or restart and try again.

Remember that some key combinations are disabled when your Mac is using a firmware password.

Learn more

See Full List On Cultofmac.com

How To Use The Mac's Mysterious Startup Keyboard Combos ...

• Learn what to do if your Mac doesn't turn on.
• Learn about Mac keyboard shortcuts that you can use after your Mac has started up.