This is a different case than the other questions about this topic, none of which helped solve this one.
I have two disks (I'll call them disk 1 and disk 2), each with its own GRUB. Both disks boot fine through the GRUB menu of disk 1, each with its own Ubuntu installation. Cool.
But as for the Ubuntu installation of disk 2, I cannot get the GRUB of Disk 2 to present the list option of booting the Ubuntu of disk 2. Running update-grub from within its Ubuntu, only finds the Ubuntu installation on the other disk. Everything else mentioned in all other questions about similar things doesn't help. Tried boot-repair auto repair, advanced repair, purging existing GRUB (which I think had no effect), you name it.
I need to get rid of disk 1 and its Grub so it really needs to work from disk 2...