If you don’t see the volume you want to use, wait a few moments for Startup Manager to finish scanning connected drives. After a few seconds, the Startup Manager appears. Use these steps to choose a startup disk with Startup Manager: Startup Manager allows you to pick a volume to start from while the computer is starting up. Temporarily change your startup disk with Startup Manager The next time you start up or restart your computer, your Mac starts up using the operating system on the selected volume.
From the Apple menu choose System Preferences.Set the default startup disk You can change the startup disk your Mac automatically uses from System Preferences.
