It turned out that by adding a second disk the drive letters changed. The disk containing the root file system was /dev/sda before but became /dev/sdb. This made it impossible for the system to decrypt the root partition.
Please note that Grub was fine, it loaded the Kernel correctly. Thus all attempts to repair the system targeting Grub were doomed to failure.
The shell that presented the error was a initramfs, not a BusyBox. It allows to to fix errors so the boot process can continue. A dedicated section in this answer explains how to do so.
Another section explains how to permanently fix the issue.
How to temporarily fix the issue in initramfs
Actually it is very simple. This answer has some more details but the steps regarding LVM were not necessary in my case.
What blocks the system to proceed to boot is that the device containing the root file system cannot be found. In my case this was /dev/mapper/ssd-root. For it to appear one has to manually open the encrypted partition:
cryptsetup open /dev/sdb2 some-arbitrary-mapname
After that, one must exit initramfs:
exit
Of course, one must use the device that contains the root file system. As map name, anything can be used. One could go with the generic sdb2_crypt, but one might choose to use a self-describing name like ssd500-crypt.
If one does not use the expected map name, e.g. sda2_crypt, one again will be prompted for a password some time later, again after a brief hang. The expected map name can be found in /etc/crypttab.
How to permanently fix the issue
In order to permanently fix the issue initramfs has to be updated. The following sequence of steps was used by the author:
- Fix the issue temporarily in
initramfs.
- Change the map name in
/etc/crypttab to something new. Anything goes, so one might use a self-describing value like ssd500-crypt.
sudo update-initramfs -u and reboot- Again temporarily resolve the issue in
initramfs. However, this time the map name must be the one used at step (2).
sudo update-initramfs -u and reboot
Possibly step (1) can be omitted if the new map name is chosen there already.
