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Determine which disk started up your Mac
Some Mac computers can be started up from a CD or DVD, a network volume, a different disk, or another operating system.
Important: If you have an iMac Pro, added security features may be set to prevent the computer from starting up from an external startup disk. See What is the Startup Security Utility?
On your Mac, choose Apple menu
> System Preferences.Click Startup Disk.
Your selected startup disk is shown at the top of the preferences pane.
See alsoChange your Mac startup disk