Bsrgvty

TL;DR

How can I boot from a EFI GRUB installation on a disc A a Windows 10 BIOS installation on a disk B?

Long explanation
os-prober is not detecting Windows anymore after having migrated Ubuntu from MBR+BIOS to GPT+EFI.

My disk situation is a little complicated:

sda 1,8T 
└─sda1 ext4 1,4T /media/Steam Steam
sdb 931,5G 
├─sdb1 vfat 512M 
├─sdb2 ext4 125G home
├─sdb3 ext4 774,5G /media/Dati_ext4 Dati_ext4
└─sdb4 ext4 31,5G Ubuntu18.04
sdc 232,9G 
├─sdc1 ntfs 100M 
├─sdc2 ntfs 59G Win10
├─sdc3 ntfs 483M 
└─sdc4 ntfs 172,9G Dati_NTFS
nvme0n1 477G 
├─nvme0n1p1 ext4 62,4G / Ubuntu19.10
├─nvme0n1p2 vfat 200M /boot/efi EFI
└─nvme0n1p3 ext4 322,1G /home home

• I have 4 disks (1 NVMe and 3 SSDs).


• Before today they were all using MBR and the UEFI was booting the NVMe in legacy mode.


• I have 2 installation of Ubuntu (19.10 and 18.04) and 1 of Windows 10.


• Each OS has it's own disk (and the 4th disk as no OS installed). This means that each OS had its own boot loader installed on the MBR of each disk.


• GRUB was also able to see every other OS so, from the GRUB of the NVMe, I was able to boot any of the 3 OSs.

Today I decided to convert the NVMe installation of Ubuntu form MBR+BIOS to GPT+EFI. Since I did not touch any other disk, I was hoping that the GRUB will detect the other OSs and boot them normally.

The conversion worked fine and I'm able to boot in Ubuntu 19.10. But os-prober recognize Ubuntu 18.04 but not Windows 10.

What I would like to do is either to find a fix for os-prober, so that it recognize Windows, or to manually specify the GRUB configuration to boot the Windows MBR+BIOS installation.

I already tried something for the second approach but it didn't worked.
I added this to the /etc/grub.d/40_custom of the EFI installation:

menuentry 'Windows 10 (su /dev/sdc1)' --class windows --class os 
 insmod part_msdos
 insmod ntfs
 set root='hd2,msdos1'
 if [ x$feature_platform_search_hint = xy ]; then
 search --no-floppy --fs-uuid --set=root --hint-bios=hd2,msdos1 --hint-efi=hd2,msdos1 --hint-baremetal=ahci2,msdos1 C08E2C498E2C3A74
 else
 search --no-floppy --fs-uuid --set=root C08E2C498E2C3A74
 fi
 parttool $root hidden-
 drivemap -s (hd0) $root
 chainloader +1

I took that from the working Ubuntu 18.04 GRUB MBR+BIOS installation.

But when I select Windows 10 from the GRUB menu I get just this error:

Setting partition type to 0x7
error: can't fine command `drivemap'.
error: invalid EFI file path.

Is it even possible to boot from an EFI GRUB a BIOS installation on a different disk of Windows 10?

Short answer, you can't boot a legacy Windows from UEFI grub. Once grub boots, the boot mode has been set to UEFI, and your Windows is in legacy mode, so cannot boot. You can select the device Windows is on (presumably with its own boot loaders), and boot that from the EFI boot menu (some function key at power-up to allow you to select boot device/OS). You could have either another grub installation or Windows boot successfully off that second disk, which would be in legacy mode. Try just booting the second disk, Windows may come up.