I remember somebody else talking about an (I think) ASUS motherboard here and saying that temporarily attached USB devices were in fact listed in the configurable boot order section of their BIOS Setup, even when the motherboard was configured for UEFI mode.  So different systems do seem to behave differently in this respect.

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jphughan - 16 November 2019 9:27 PM

I remember somebody else talking about an (I think) ASUS motherboard here and saying that temporarily attached USB devices were in fact listed in the configurable boot order section of their BIOS Setup, even when the motherboard was configured for UEFI mode.  So different systems do seem to behave differently in this respect.

I didn't get a definitve answer from Dell, Microsoft, or here. With the help of an independent consultant, I was able to boot the USB device.