Here's an update. I have finally managed to see the purple screen with the keyboard icon at the bottom and then I even see the full menu with all five choices...
However, my system then reboots before I can enter a choice, and so it loops, endlessly, as long as I have patience to press keys!
I would be most grateful if anyone had any ideas about how to stop the autoreboot. By the way, I'm still getting the "Waiting for disks to settle..." error message. I've tried it with just a SATA hard drive, just a SATA SSD, and with both and a SATA DVD drive. I got this far by specifying a boot from legacy USB key. I haven't found a way to get my bios to access the HD and SSD by legacy instead of UEFI. Maybe that's the problem.