I also hit this today with the latest upgrade. On a system that has been upgraded many times in the past. It's 2.02+dfsg1-20+deb10u4 upgrade this week caused it to fail to boot. It's was installed in 2014 and upgraded continuously since then.
root@euphoria:~# ll /var/log/installer/syslog -rw------- 1 root root 281561 Jan 6 2014 /var/log/installer/syslog root@euphoria:~# grep 'Installing grub on' /var/log/installer/syslog Jan 6 21:12:29 grub-installer: info: Installing grub on '/dev/sda' Initially installed with one disk in degraded RAID mode and then subsequently additional disks were added. grub-install was used to install grub to the new disks. There are two ways to use the debian-installer in rescue mode. One is to execute a chroot to the installed system and then install grub from there. In that mode there are two ways to update grub. Boot rescue mode. Using the debian-installer directly: 1. Reinstall GRUB bootloader 2. Execute a shell in /dev/sdX 2.1. grub-install /dev/sdX 2.2. dpkg-reconfigure grub-pc The first way updates the disk grub image okay but does not update the debconf settings. The second way updates the disk grub image okay but does not update the debconf settings. Only the third way updates the disk grub image okay and also updates the debconf settings. Due to the use of /dev/disk/by-id/... I can also imagine that replacing a failed disk would also create another situation where grub bits are updated but debconf is not kept in sync. I am sure that accounts for several failures I have experienced in the past but did not understand the root cause of why at the time. The root cause of this bug appears to be the abuse of debconf as a registry. I always thought that was explicitly forbidden. It's clear from this bug that debconf is required to be up to date and in sync or upgrades will fail. At the least it should be strongly documented that grub-install does not update debconf and the package relies upon the debconf configuration for operation. I was completely unaware of this requirement before today. I really think the grub package should declare a config file (that is not debconf) for what devices need updates. Then it would be plain and simple to the local admins what files need to be updated to allow grub to automatically upgrade. Bob