Hello!
Thank you for your answer. Unfortunately I have had to give back NUC to my customer (they borrowed it to me for test reasons), so I can't test on it anymore.
However, I'm almost sure I already took that action, but didn't work.
I adopted the following workaround (to install Win 8.1 Pro UEFI):
- Activate UEFI and Legacy boot
- F10 Menu
- Selected UEFI boot via USB Optical device
Conclusions:
USB UEFI Optical device work, instead USB UEFI pendrives don't work.
USB UEFI Optical device works only if UEFI and Legacy boot are both activated; if I disable "Legacy", it disappears from F10 Menu.