I am not from Intel. Have you double-checked the BIOS boot settings on the failing NUC, and if so, have you considered something like reset BIOS settings to default and try again?
My early DN2820FYKH had some sheet of instructions about settings in BIOS relating to booting, I think it was something like Windows 8 vs other operating systems. There are often settings in BIOS about whether to boot Legacy and/or UEFI, and often secure boot settings. I think the instruction sheet said to disable UEFI unless you were running Windows 8 - Maybe your failing NUC has UEFI disabled?
It's not "a" failing NUC, it's a bunch of failing NUCs - but some actually work as expected. All of them have been updated to the same version of BIOS with the same settings, although it's not clear that the BIOS update process actually updates everything that should be updated, so there might be some stale data left over from the old BIOS. There has been no Windows anything on these devices, they're fresh out of the box.
Unfortunately we need UEFI, so disabling it isn't really an option.
Sorry if unclear, I was saying that early NUC DN2820 BIOS settings may have disabled UEFI based on something they called "OS Selection." I thought that was how they initially supported some Linux on that model, so I just wanted to make sure that the failing NUCs had UEFI enabled in the BIOS settings.
Yes, UEFI is enabled. In theory the config on all machines is identical - fresh out of the box, same BIOS and same BIOS config installed from the same thumb drive, and bit-for-bit identical hard drives.