There is a number of actions you are able to take by holding a key or a key combination during the start up of the Mac computer running macOS or OS X.
Press and hold the keys immediately after pressing the power button on your mac or after your Mac restarts until the required function takes place.
|Hold during start-up||Description|
|Shift||Start up in Safe Mode.|
|Option||Select startup disks|
|C||Start up from an available CD, DVD, or USB thumb drive with macOS/OSX|
|D||Start up from the built-in Apple Hardware Test or Apple Diagnostics utility|
|Option-Command-P-R||Reset NVRAM or PRAM.|
|Command-R||Start up from the built-in macOS Recovery system.|
|Option-Command-R or Shift-Option-Command-R||Start up from macOS Recovery over the Internet.|
|Command-S||Start up in single-user mode.|
|T||Start up in target disk mode.|
|X||Start up from your macOS startup disk when it's not the default|
|Command-V||Start up in verbose mode.|
|Eject key OR F12 key OR mouse/trackpad key||Eject removable media, such as an optical disc.|