There are several reasons why the Legacy Boot setting in the BIOS might be grayed out and can't be enabled. You may need to check each option below before you can enable Legacy Boot.
First, be sure to check if your Intel® NUC model supports Legacy Mode as per article (000057401) - Intel® NUC Products Phasing Out Legacy BIOS Support.
|Why are you seeing it?||How to fix it|
Modern Standby is enabled in BIOS.
Legacy Boot isn't allowed when Modern Standby is enabled.
Change the Sleep Type Support to Legacy S3 Standby:
At the next restart after changing this setting, you'll see this message
WARNING! A change has been detected in the BIOS Sleep Type support setting, from <last sleep type> to <new sleep type>. Dynamically changing the Sleep Type support setting between Legacy S3 Standby and Modern Standby is not supported and may require reinstalling the Operating System. Would you like to continue? Enter (y) for Yes to continue, or (N) for No for reboot.
According to Microsoft, switching the power model isn't supported in Windows without a complete Windows reinstall. Changing this BIOS setting doesn't change the Windows power model.
Secure boot is enabled in BIOS.
Legacy Boot isn't allowed when Secure Boot is enabled.
Disable Secure Boot:
SATA Mode is set to Optane Mode in BIOS.
Legacy Boot isn't allowed when in Optane Mode.
Change the Chipset SATA Mode:
Warning: The operating system might not boot if this setting is changed after the operating system has been installed.
1To find out if your Intel NUC has the Visual BIOS or the AptioV BIOS, see BIOS Settings Glossary for Intel® NUC.
|Modern Standby (Microsoft site)|