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.
- Select your startup disk, then restart your Mac.
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.
- Hold down the Option key immediately after turning on or restarting your Mac.
- Release the Option key when you see the Startup Manager window.
- Select your startup disk, then click the arrow or press Return.
If your Mac is using OS X Lion v10.7.3 or later, you can also use this method to start up from your Time Machine backup disk. Startup Manager identifies your Time Machine backup as ”EFI Boot.”
If you can't select your startup disk or start up from it
Some third-party external devices use Option ROM firmware. To enhance system security, Mac computers with up-to-date software don’t automatically load Option ROM firmware. As a result, your Mac doesn't see devices that have Option ROM firmware until you load the firmware by pressing Option-Shift-Command-Period at the Startup Manager window. Do this each time you want to start up from the device or from a startup disk connected to it.
If you're using an Early 2015 (or older) Mac model, you can disable this security feature and allow your Mac to load Option ROM firmware automatically. This removes an important protection against potential unauthorized access to your Mac by other people with physical access to it.
- Open the Terminal app, which is in the Utilities folder of your Applications folder.
sudo nvram enable-legacy-orom-behavior=1, then press Return.
To undo this command, enter
sudo nvram -d enable-legacy-orom-behavior.