@smeg It sounds like the issue might be that you are running them on separate drives, as opposed to partitions. You may have two installations of Grub.
You might have to run update-grub on the older installation in order to re-detect everything because grub will detect the installations on other drives but the second one did not yet exist at the time it was installed on the first drive.
And, it all depends on which drive the computer is actually booting from. If you have two versions of grub, then the one that launches is the one that the computer boots up from. If that one hasn't detected what is on the other drive, then you won't see it.
@smeg It sounds like the issue might be that you are running them on separate drives, as opposed to partitions. You may have two installations of Grub.
You might have to run update-grub on the older installation in order to re-detect everything because grub will detect the installations on other drives but the second one did not yet exist at the time it was installed on the first drive.
And, it all depends on which drive the computer is actually booting from. If you have two versions of grub, then the one that launches is the one that the computer boots up from. If that one hasn't detected what is on the other drive, then you won't see it.