Here are our recommendations:
- Make sure that the USB drive is a bootable device.
- Test with a a different device if possible to narrow the issue since could be related to compatibility.
The problem appears to be the presence of a NETWEAR USB WiFi dongle in a USB2 slot. Removing the dongle allowed the discovery of the USB3 DVD drive to succeed.
FWIW, are there any USB DVD drives that do not qualify as a bootable device? Otherwise, that's exactly was my problem/.