[DrTiTus]

I just spent a week trying to get an older (2013? not that old) PC to boot Windows 10 after install. Linux worked fine (of course), and I have an "identical" (newer BIOS) machine that has no issue. BIOS wouldn't even list the hard drive, tho it showed on the SATA port which was new to me. I tried all sorts, including several failing BIOS upgrades, Acer technical support, grub commands to work around it and all sorts of bcdboot repair commands, 3rd party disk conversion tools and Linux dual boot hacks to try and make the friggin thing work properly.

Turns out the cause was that I chose the default UEFI option when I initially booted the Windows install USB stick (causing a GPT partition table to be made which the BIOS doesn't like - it's MBR now). BIOS still won't update, Acer tells me it's broken, but I no longer care as it works perfectly now. What a cunt of a time.

TLDR: UEFI is the devil on older machines. Don't choose the UEFI option even if it happens to be listed first and appears to be no different, unless you have good reason.