Change your Mac startup disk
You can make your Mac start up from a CD or DVD, a network volume, a different disk, or another operating system. To do so, you change your startup disk.
Important: If you have a Mac with an Apple T2 chip, added security features may need to be set in order to change the startup disk. See What is the Startup Security Utility?.
Change your startup disk once
Press and hold the Option key as you restart your Mac.
When you see the available startup disks, select one.
If your computer is on a network and a network startup disk is available, press and hold the N key as you restart the computer to start up from the network startup disk.
The next time you restart your computer, it starts up from the disk selected as your startup disk in System Preferences.
Change your startup disk for every startup
On your Mac, choose Apple menu > System Preferences, then click Startup Disk.
Click the lock icon to unlock it.
Click the icon of the disk you want to use, then click Restart.
WARNING: When selecting a network startup volume, make sure you select a network startup volume and not a network install image. Choosing a network install image reinstalls your system software and may erase the contents of your disk. A standard network volume icon appears as a globe with a folder. A network install icon appears as a globe with a downward-pointing green arrow.