If GRUB gets wiped by other operating system or is installed in the wrong disk, do this:
-> Boot from a live Debian USB
-> Start a console/terminal.
-> Find the disk where you want to install GRUB (sdX):
# lsblk -f
-> Create a mountpoint (/ospart) where you are going to mount your system partitition (sdXY)
# mkdir /ospart #
mount /dev/sdXY /ospart
-> Change root to the system partition (/ospart)
# for i in /sys /proc /dev; do mount --bind $i /target$i; done #
chroot /ospart /bin/bash
-> (Re)Install GRUB on your desired disk (sdX) #
grub-install /dev/sdX #
update-grub
> Reboot!
bytebin 