Richard, as you didn't use 'apport' or 'ubuntu-bug' when creating this bug report I'm unsure if what you are reporting applies to me or not as your report doesn't include any version information.
I have Ubuntu 18.04 and Ubuntu 19.04 installed on this laptop which has two disks /dev/sda and /dev/sdb. Using Ubuntu 18.04 I can install grub, update grub and boot to either installation without any problem. >From Ubuntu 18.10 (now upgraded to Ubuntu 19.04) any attempt to update grub seemed to fail as I could only boot to the existing kernel and not to the newly installed kernel. I have seen this on more than one occasion. If this is an example of what you are reporting and something that has been an issue since the 18.04 release then I can confirm the bug report. There's a reference to my issue at: https://ubuntuforums.org/showthread.php?t=2403574&p=13808168&viewfull=1#post13808168 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1803805 Title: grub-pc fails to install on the last of multiple devices To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1803805/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs