said by aguen:Ok, which was the last OS installed?
Ubuntu
said by aguen:If it was one of the *nix distro's, what option did you choose for the location of the grub boot loader?
The Ubuntu installer recognized sda1 and sda4 as efi partitions. I made no manual selection for configuring grub.
said by aguen:Do you currently have only Windows 10 installed and none of the earlier versions?
Yes. And I've never installed any other Windows on this drive.