If pressing the Option key at startup doesn't bring up the Startup Manager on Mac, check the following tips:
① If your Mac has the Apple T2 Security chip, you need to allow booting from external media in Startup Security Utility.
② If your Mac is Intel-based, you need to turn off your Mac, then press the power button and immediately hold down the Option key until seeing available startup disks.
③ If your Mac has the Apple Silicon chip, you need to shut down your Mac and press and hold the power button until "Loading startup options" shows up rather than the Option key to access Startup Manager.
If you still can't select a startup disk, try to change the startup disk from System Preferences/Settings.
On macOS Ventura and later:
- Keep your bootable drive connected.
- Go to the Apple menu > System Settings.
- Click General > Startup Disk.
- Click the icon of the disk you want to use and click Restart.
On macOS Monterey and earlier:
- Keep your bootable drive connected.
- Open the Apple menu and select System Preferences.
- Tap the yellow lock and enter your Mac's password.
- Select a disk and click Restart.