Grub not showing Windows 10 entry

Question

After installing Windows 10 I installed Debian 9 in a different partition. I wanted to have dual boot. However something went wrong and currently only Debian is displayed as unique OS in the selectable start list. These are the partitions:

• sda1: EFI system

• sda2 and sda3: Windows 10

• sda3: Debian

How can I make the two OSs to be shown in the list?

Things I tried:

1. Installing GRUB in the MRB instead of in the sda1 as UEFI
2. Use the tool Grub repair

After 2 (at some point) two entries of Windows were displayed in the GRUB. However I could only see the wallpaper of Debian but no trace of the login box where to insert my credentials.

Update Currently I broke the grub. At start up there is a black script with the message:

error: file '/grub/i386-pc/normal.mod' not found. grub rescue>

I followed the following post

However normal.mod is not inside the folder grub:

grub rescue> ls (hd0,msdos6)/boot/grub/

unicode.pf2 x86_64-efi/ locale/ fonts/ grubenv grub.cfg

Answer 1

Grub reads it's boot configuration from /boot/grub/grub.cfg, which is generated by grub-mkconfig. grub-mkconfig reads /etc/defaults/grub for configuration variables, executes the files in /etc/grub.d and writes the output to the boot configuration.

One of the files in /etc/grub.d should be named 30_os-prober. That file, if it exists, should call os-prober (if it's not disabled in /etc/defaults/grub).

So, to find out, where stuff goes wrong, check if os-prober works and outputs something like /dev/sda1:Windows 10:Windows:chain (that's my EFI partition).

If it does, check if os-prober is disabled in /etc/defaults/grub and that /etc/grub.d/30_os-prober exists.

You could also manually add the entry in /etc/grub.d/40_custom. The Arch Wiki has good information on how to do that.

Answer 2

First boot into debian and mount windows 10 partition. Then in terminal run sudo os-prober && sudo update-grub. Check if problem solves.