That makes sense but, as I recall, when I plugged that APFS SSD in to my High Sierra 2011 Mac, an icon DID appear on the screen. That's why I assumed I could back up to it. And when I commanded SuperDuper to back up to it, it ground and ground for twenty minutes and pretended to be backing up. It was only when I tried to boot from that backup twenty minutes later that the SSD didn't show in the Boot Manager. That is, the Boot manager knew that it wasn't a compliant disk, but SuperDuper didn't seem to be aware that it wasn't.
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