With the release of macOS Monterey, Apple has changed the way you set a Secure Boot policy for bootable external disks on M-Series—M1, M1 Pro, M1 Max, or M2—Macs. Here’s what you need to know to successfully boot macOS from an external drive.
Full Security is the default setting; as the name suggests, it offers the highest level of security. (This is a level of security previously available only on iOS devices) You have to use the “Set allowed boot media” to control whether your Mac can start up from external or removable media.
The default, most secure setting is to disallow that capability. If you attempt to boot from such media and you get a warning that your security settings do not allow it, you can change the setting in Startup Security Utility.
To drop from Full Security, you have to start up from the external disk, shut down, then start up in Recovery mode by holding the Power button.
That takes you into the Recovery system on that external disk, where you can change its policy using Startup Security Utility. If you’re using a Mac with the Apple T2 Security Chip, the tool offers three features to help secure your Mac against unauthorized access: firmware password protection, Secure Boot, and the ability to set allowed boot media.
Also, note that when started up from the Recovery system on the internal SSD, you can only set policy for that disk, not external disks.