There are multiple reasons why the Legacy Boot is greyed out in your system BIOS. The following suggestions will help you fix the problem. Before you proceed, if you are using Intel NUC products, you ...

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", ...