[gbdoc]

My main theory is that when I tell my Mac to boot from the external drive, it shuts down briefly and loses power before the re-boot begins. I assume that’s normal behavior. Upon regaining power and beginning to re-boot, the Mac responds more quickly than the external drive, so that’s the one which boots. I first suspected that that happens to me because my external is an HD, which may take a bit to spin up to speed, but mmurray, above, has the same result using an SSD, which is faster, so that doesn’t seem to be the issue.

Your answer suggests that neither an SD nor any other clone can’t contain the OS. That means to me that SD clones are not now nor will ever will be bootable, unless the 3rd-party utility would/could also install the OS on it. Correct? Is that do-able? Sounds like something you might consider for SD! 4. 🤞

My other theories boil down to the certainty that there is another explanation which only better-versed people could understand, which may or may not be correctable.

But, Dave, before I start to fool around with my backup drive (installing the OS on it), which contains my SD clone and a Time Machine, and works fine otherwise and is the only one I have, I think I’ll wait until you get an M2 Mac yourself (you will, sooner or later, I’m sure) and try this out, and report on your results.