can't external boot

[Dan Lester]

Bingo! It's fixed. And many thanks to you for the clue, Dave.

Here's the deal. You asked if my keyboard was plugged directly into my Mac. No, my keyboard was plugged into a new powered USB hub. Now, I know that such hubs can mess with things, and certainly a Bluetooth keyboard won't be read properly on boot. So I plugged the keyboard DIRECTLY into the Mac. It works! (Just like it used to.)

This is an important thing for people to know about when doing bootable backups. That a USB hub may prevent the OPTION key from opening up the Startup Manager.

ALWAYS ALWAYS ALWAYS plug your keyboard directly into the Mac. That instruction ought to be engraved on Mac keyboards.

So you were exactly right that the OPTION key wasn't getting read at startup. But the question was why it wasn't getting read if it was working at other times!