Well...
This sounds like a guessing game.
So...
1- I'm guessing, this is a UEFI setup.
2- I'm guessing, since you posted from some Ubuntu derivative, it shows up in Ubuntu grub menu.
If my guesses are correct, boot into Debian, run command: fdisk -l to find which device it's boot directory is in, if it don't have it's own /boot partition it will be in the root of Debian. So if Debian root is for example: /dev/sda4, run command: grub-install /dev/sda4 as root or sudo from within Debian.
That should rebuild the boot files in the EFI system partition and add it to the firmware menu.
That would be the quick easy way, from Ubuntu or Debian you can have a look in /boot/efi/EFI to see if there is a Debian directory, in which you could take the long route and try adding Debian to the menu with efibootmgr commands.
If I guessed wrong, please post relevant information.
Last edited by Brains; 03-21-2019 at 12:22 PM.
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