I have researched for this question, but have not been able to find a clear answer... or even a clear question. So I will use a simple example.My computer have **two hard disks:**  Hard Disk 1: Ubuntu Linux with GRUB2 on MBR.  Hard Disk 2: Any other operating system (does it bother which one?) with some other MBR loader.  GRUB2 only boots to Ubuntu (on 1st hard disk, in my example).Issuing update-grub does not detect the operating system on the 2nd hard disk, but I know there is one.If I enter the BIOS on boot time and change boot order to Hard Disk 2, the second operating system boots OK.How can I add an entry to GRUB2 that boots to the second hard disk?(The fact is: I think this question can be asked without specifying which one is the operating system in the 2nd hard disk. Or not?)Adding upon request output of os-prober for a sample machine with this configuration:luis@threepwood:~$ sudo os-prober/dev/sdb1:Windows 7 (loader):Windows:chain/dev/sdb7:Ubuntu 14.04.3 LTS (14.04):Ubuntu:linuxIn this particular example (I changed boot order at BIOS) case:  sdb is HDD-1.sda is HDD-2.
GRUB2: boot to a second (another) hard disk
grub2;dual boot