Dear Graham,The new laptops (notebooks) or desktops (desktop switches), with Intel 64-bit (X64) chipstet, come with a new boot system called UEFI (Unified Extensible Firmware Interface). It is a faster and safer way to boot the Operating System that prevents third parties from installing a new version of Windows or from accidentally formatting the partition where Windows is installed.
in primary boot, leave it in the same order:
USB with media
HD
Always leave the item from where it will be installed first and right after the HD, the rest does not matter.
And try switching to legacy again, if the error appears again, please report the error in detail to facilitate assistance.
Thank you in advance!