Known issue with ubuntu-16.04.3-desktop-amd64
May need to install os-prober to enable multi-boot on Ubuntu.
Have a look at your Ubuntu’s /boot/grub/grub.cfg
Also use terminal to check your partitions, Example command: sudo gdisk /dev/sdb
Found openSUSE was ignored, not included in the grub.cfg then when install Ubuntu OS.
Need a change in Ubuntu - believe installed os-prober to enable multi-boot.
When done Ubuntu recognized and allowed selection of my openSUSE boot partition.
From time do upgrade Ubuntu with clean install, so need again re-organise my openSUSE root partition.
Earlier, found when boots to Ubuntu, by editing grub so add correct boot address for openSUSE 15.2 it starts ok.
Example boot addresses
**set boot=‘hd0,sda2’ **
openSUSE use more, so so boot into openSUSE then did the grub2-mkconfig -o /boot/grub2/grub.cfg
Result is openSUSE is my default boot, with option to select Ubuntu.