The Mac Mini would not boot from external disks that ran software other than the factory configuration. This disables a vital feature for us. This is because of the Apple T2 security chip. It took some time to figure out how to overcome the problem. Here's a link to the solution with image: https://discussions.apple.com/thread/253199493?login=trueMethod: I booted into recovery mode. I selected the Secure Boot option and then turned off full security settings & selected the option 'Allow booting from external or removable media. Now I can boot from external SSD disks. Apple is incrementally eradicating user control and customisation.This 'security' obsession Apple has adopted takes away autonomy and the ability to customise from the user, sadly a now increasingly more pushy trend at Apple. It's as if they have decided to take the notorious  Microsoft route where its users have to constantly battle against what the community sarcastically identifies as Microsoft 'features'. Come back Steve Jobs!!  Anyway it's fixed and the Mac mini  is integrated with the other Macs here and can now boot from any external ssd disk. - A versatile and well-priced Mac apart from that problem.