For various reasons related to chaining a custom grub loader to Windows 10 and an additional grub loader to Linux, I tend to choose Legacy boot for its simplicity to achieve this.
Both my Boot->UEFI Boot [checked] and Power->Secondary Power Settings->Native ACPI PCIe Support [checked] options are enabled (and grayed) and can't be disabled. I've already adjusted Boot->Secure Boot->Disabled as required for the first gating option for enabling Legacy Boot. What is required with the 0054 BIOS version on the NUC9 it order to switch to Legacy Boot?
I'm assuming that having grayed options suggests the feature is supported but a higher priority setting is gating me. (This version doesn't have an option for "Modern Standby" as suggested elsewhere on the forums.) I've confirmed that the boot process isn't doing any "auto detection" as it only offered to boot a Linux recovery USB in UEFI despite my USB image being configured with both.
Any suggestions to enable Legacy Boot?
Paul_K_Intel1, Thank you for posting in the Intel® Communities Support.
In reference to your question, to try to provide the most accurate assistance, we will do further research on this matter, for us to be able to do that, please provide the SSU report so we can verify further details about the Intel® NUC:
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Intel Customer Support Technician
A Contingent Worker at Intel
These models (QN/QX) do not support Legacy Boot at all. Most new NUCs have not nor will support Legacy Boot going forward. Since Modern Standby is becoming more common, this will be the case with most SI or OEM builders in the future too.