I have just received new NUC7CJYH, the first thing I noticed - there is no legacy boot option in BIOS. It does not recognize any of the legacy drives attached, and when I go to Visual BIOS there is a "Legacy Boot Priority" panel but it is empty. I cannot disable UEFI boot, checkbox is active but greyed-out (see attachment).
I flashed latest BIOS JYGLKCPX.86A.0027.2018.0125.1347.
Does it mean booting from legacy drives is not supported anymore?
I am not from Intel and I do not have that model, but page 28 of the manual says Legacy OS are not supported, and page 56 of the manual says no Legacy USB Support. Have you tried a 2.5" SATA instead of USB? My guess is that won't make difference, because a search turned up a very recent review of that NUC with a YouTube video showing the various BIOS screens with a SATA drive connected and it looked like your screen (no obvious Legacy boot).
It is currently the top one in https://www.intel.com/content/www/us/en/support/articles/000005545/mini-pcs.html Technical Product Specifications for Intel® NUC Products
I read Specs doc, I'm still confused. Note that I did not ask about legacy USB support - I don't care about it and Visual BIOS clearly misses any option related to legacy USB. But Visual BIOS has a panel for legacy boot priority which sort of implies that booting from legacy drives should be supported. But it does not recognize any of my legacy HDDs, ether in SATA or over USB. Note about Legacy OS is also confusing, you can install modern OS on legacy (non-EFI) disk.
The Bios graphics may include legacy boot panel(as graphic is standard for all NUCs), however if the specification defines that legacy USB is not supported I understand that if you insert into USB slot a legacy device (i.e. device partition with MBR partition), such device will not be detected by the system and therefore you can not boot from such device. So, if you will have Windows 10 on your USB which was partitioned with MBR partition, it will be not recognized. You need to use media (USB, HDD,...) partitioned with UEFI partition (i.e. GPT partition). During the POST the UEFI Bios scans all of the bootable storage devices for a valid GPT. Unlike the MBR, a GPT does not contain a boot loader.
Is your bootable device connected to USB socket? So, if your device has MBR partition, it will be not recognized. If you will change it to GPT, you will be able to boot from this device.