I’m having Ubuntu 15.10 dual boot Window 10 on the same hard disk.
I have Ubuntu on 2 ext4 partition (mount point / and /boot). I also have 4 other ext4 partitions (which will be used to install openSUSE, Linux Mint, Korora, Manjaro) and one swap partition. My grub was install into this hard disk when installing Ubuntu and now I want to install openSUSE into this hard disk without replacing Ubuntu’s Grub (without installing openSUSE’s Grub), how to do this? I have installed openSUSE 13.2 and I have replaced Grub of other Linux OS, I don’t want to have the same.
Obligatory question - are you using legacy BIOS or UEFI to boot?
I’m using BIOS!
In this case you should install bootloader on partition and make sure “Install generic MBR” is unchecked. On openSUSE installation summary go to bootloader and double check what is proposed. After installation of openSUSE you will need to boot into your primary OS (I suppose, Ubuntu) and update bootloader there so it picks up kernel from openSUSE.
Alternative is to manually create grub configuration snippet in Ubuntu that simply chainloads openSUSE grub from partition. That way you will be sure to always have actual boot menu in openSUSE. This can be done as second step once you are sure everything works 