Comment 1 for bug 1786992

To clarify, if Ubuntu is booted in EFI mode, os-prober will currently assume that grub will continue to boot in EFI mode and so refuse to add a bios Windows chainloader stanza to the menu since it will not work. It will only add a windows entry if it finds the windows efi loader in the ESP. It may be able to come up with sufficient logic to have a bios chainloader entry that is only displayed by grub-pc and hidden in grub-efi, and a corresponding EFI mode that is hidden vice versa.

That would get us out of this mess at least with Windows, though bare metal FreeDOS installs are still going to be left out in the cold.