It turns out that mdadm had been uninstalled some two weeks prior to the most recent upgrade and so grub-install had no user space tools to query the RAID arrays. Reinstating mdadm fixed the problem.
** Changed in: grub2 (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1759877 Title: grub-pc fails to upgrade with raid disks To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1759877/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs