By default, your Mac starts up from its built-in hard disk, but a startup disk can be any storage device that contains an operating system compatible with your Mac. For example, if you install macOS on an internal or external drive, your Mac can recognize that drive as a startup disk. You can then follow the steps in this article to start up from it.
Use Startup Disk preferences
When you use Startup Disk preferences to select a startup disk, your Mac starts up from that disk until you choose a different one.
- Choose Apple menu > System Preferences, then click Startup Disk.
- Click the lock and enter your administrator password.
- Select your startup disk, then restart your Mac.
If you see a message that your security settings do not allow this Mac to use an external startup disk, check the External Boot setting in Startup Security Utility.
Use Startup Manager
When you use Startup Manager to select a startup disk, your Mac starts up from that disk once, then returns to using the disk selected in Startup Disk preferences.
- Press and hold the Option (Alt) key immediately after turning on or restarting your Mac.
- Release the Option key when you see the Startup Manager window.
If your Mac is protected by a firmware password, you can release the key when you're asked to enter the password.
- Select your startup disk, then click the arrow under its icon, or press Return.
If you press and hold the Control key during this step, your selection is saved in Startup Disk preferences, so it persists until you change it.
If you can't select your startup disk or start up from it
Check for these possibilities if you can't see your disk in Startup Disk preferences or Startup Manager, or your Mac won't start up from it.
Check for a compatible operating system on the startup disk
Check startup security settings
Check for Option ROM firmware
If you're in Startup Manager and can't see a third-party startup disk, the startup disk could be using Option ROM firmware. To enhance system security, Mac computers with up-to-date software don’t show devices that use Option ROM firmware until you load their firmware. To do that, press Option-Shift-Command-Period while in Startup Manager. If your startup disk appears, do that each time you want to start up from it or from another disk connected to it.
If you're using a firmware password, the ability to load Option ROM firmware is disabled as an additional security protection.