... "possibly option-booting would have found it" ... I guess I was groping in that direction too; right after I got the ?/envelope response I tried to boot again with OPTION held down; but this time, instead of being offered
both the internal HD
and the ext HD which SD! had created earlier, as had once been the case... after I'd restored (incorrectly) to the int HD, OPTION-booting offered me
only the ext HD which I had been booted from while restoring that same image to the int HD.
But now we'll see, won't we... one hour down, a couple-ish more to go...