Why doesn't GRUB find Windows?

Question

GRUB

Xubuntu (disk partition contains uefi partition) and Windows 10 (with non-UEFI MBR partition) are installed on two separate physical disks. When you enter the command sudo update-grub does not find windows. Windows OS was found in grub Customizer after editing /etc/grub.d/custom_40 file:

menuentry "Windows 10" {
insmod ntfs
set root='(hd1,XXXXXXXXX)'
search --no-floppy --fs-uuid --set XXXXXXXXX
chainloader +1
}

The OS selection menu appears on boot, but when Windows is selected, it says:
Ошибка: invalid efi file path
sudo os-prober does not help.
It used to be possible to find both OSes with sudo update-grub when Ubuntu was installed without a uefi partition. How to fix this situation safely without reinstalling systems?

Answer 1

Both OSes must boot either through EFI or without, otherwise GRUB will not see Windows.