Non-UEFI, not secure boot, Legacy boot setup. Two Samsung 860EVO drives. One has Windows 10, the other Kubuntu 18.04. The NUC will not default boot to the Kubuntu SSD, claiming it's not a bootable device. However, if I select the boot menu from BIOS and choose the drive, it boots immediately to GRUB.
BIOS version is still 0058 but I see not notes about fixing boot issues in the update d BIOS notes so I haven't bothered to update it.
I tried pulling the Windows drive completely but then I spent 10 minutes trying to get back to BIOS. It simply said "No bootable device found" regardless of which SSD was installed or in which slot. Eventually by putting the Windows SSD alone in the first slot the Boot selection menu (F2, F7, F10) appeared again and it booted to the Windows drive.
I love the form factor but I'm not impressed with this NUC.
If you aren't running on the latest available BIOS, you aren't going to know whether issues have been handled.
If nothing else, at least tell us which NUC model we are talking about (there are a great many).
If this is Hades Canyon (NUC8i7HNK/NUC8i7HVK), support for Legacy boot has been removed from the BIOS; you *must* use UEFI boot.