If you have a new Mac, one that includes the
T2 chip, you will have a default setting that may prevent you from booting from an external drive. I think that chip is in the new Mac mini, and MacBook Pro. There will be others as the new models are released. The purpose of the T2 chip is to lock down you Mac even more from malware and hardware hacks.
Many have complained that they cannot boot into Linux on their new Macs. While reading the discussion at TidBITS, I came across info of the
Startup Security Utility. There is a section on that window that shows a setting that is normally set that may be causing problems when trying to boot from an external drive.
So far, the only way I have found to access this utility is through a Recovery restart. There, you will find the "Utilities" menu and under that will be the "Startup Security Utilities" item.
If you
don't have the T2 chip, all you will probably see is an option to set a hardware password which you will see before the normal log in screen. That is usually off by default.
If you do have the T2 chip, you will see a much different Startup Security Utilities window. That is where you will find the settings shown in the attached image. The settings shown in that image will have the "External Boot" set to "Disallow..." by default! That is fine, except you may not have been aware of that.
In my humble opinion, the language says that you can not boot from an external drive no matter what OS might be on it. While it does not explicitly say that, that's the way I interpret it, but I've only been speaking English for ~74 years. I hav ben rong beefour, houevr!
All I am saying is that we may have to start asking if a questioner has a T2 machine or if they are unable to boot from an external drive that they think they should be able to. Just another detail that can cause inconveniences when trouble-shooting problems...