You may see the “Secure Boot can be enabled when System in User Mode” error message while enabling the Secure Boot on your device. If you want to install Windows ...
My MSI Z97 motherboard (updated to the latest bios) seems to regard all of my bootable devices - SSD, HD, DVD-RW, flash drives - as legacy only. Specifically, if I switch to "Windows 8/8.1/10 mode", ...