Change the Mac Startup Drive on Boot
A Macs startup drive can be changed during system boot by holding down the OPTION key, start holding down Option the moment you hear the boot chime sound and while the screen is still black, this will bring up the boot drive manager.
From this startup boot disk menu, you can use the mouse or the arrow keys to select the drive or partition to boot from, and then either click or hit return to startup the Mac from the chosen boot volume.
How to Change Startup Drive on Boot for a Mac
The simplest way to access the Mac startup manager is by holding down the Option key during system start. You can initiate this process after the Mac has either rebooted or is starting up cold from being turned off.
Holding the Option key on system boot will bring up the startup drive menu on any Mac, that is how you change the boot volume on system start
This allows for a per-boot one time selection, and selecting a boot drive from this menu will not change your default boot drive. For future boots, you can either continue to hold down the Option key to access this menu, or you can change the default startup disk directly from System Preferences if you’re looking for something more permanent.
Once you hold down the Option key, you’ll be presented with the boot menu which shows all available boot drives either in the Mac or connected to the Mac:
This should probably go without saying, but in order for the multi-boot startup menu to appear and work as intended, the Mac must have multiple bootable volumes. This can be the primary Macintosh HD partition and the Recovery partition, or the primary Mac HD partition, a secondary OS X installation, a bootable installer drive, an external drive, or any other boot volume.
We’ve used this many times to change boot OS versions with dual-boot configs, from macOS Mojave, High Sierra, MacOS Sierra, Mac OS X 10.11 El Capitan, 10.10, 10.6 to 10.7, 10.7 to 10.8, and 10.9 Mavericks. If interested you can read more about creating and booting between multiple OS X installations, check here for Lion & Snow Leopard, and here for dual boot configs for OS X Mavericks and 10.8 Mountain Lion, or even dual booting OS X El Capitan and Yosemite or Mavericks.