macOS User Guide
- Work across devices using Continuity
- Use iPhone as a webcam
- Use iPhone with Desk View
- Stream audio and video with AirPlay
- Use one keyboard and mouse to control Mac and iPad
- Hand off between devices
- Unlock your Mac with Apple Watch
- Make and receive phone calls on your Mac
- Sync music, books, and more between devices
- What is Family Sharing?
- Set up Family Sharing
- Add a user or group
- Set up Screen Time for a child
- Share purchases with your family
- Watch and listen together with SharePlay
- Share a Photo Library
- Collaborate on projects
- Find content shared with you
- Find your family and friends
- Play games with your friends
- Resources for your Mac
- Resources for your Apple devices
Determine which disk started up your Mac
Some Mac computers can be started up from a network volume, a different disk, or another operating system.
Important: If you have a Mac with an Apple T2 Security Chip, 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 Settings, then click General in the sidebar. (You may need to scroll down.)
Click Startup Disk on the right.
Your selected startup disk appears selected.