System restore from a broken Ubuntu

I have an ubuntu system (dual booted with windows) that shut down during an upgrade from 20.4 to 21.10. It does not manage to boot.

I can enter recovery mode and drop into root shell. I have an external hard drive with a restic system backup from right before starting the upgrade (done as described in the docs). I want to restore my system.

I have no experience with restoring back ups. I tried simply mounting the hard drive to /mnt/restore and then running restic restore latest / on the repo. In the middle of a restore, it start outputting errors about “transport endpoint not connected” like so:

I currently cannot connect to the internet.

How should I run this backup? Is there something I did wrong, or is this a different problem.

(I also have an additional separate backup with all of my personal data and most of my configurations. In the worst case, I will reinstall ubuntu or another dist and start from there)

Which filesystem does your external hdd use? Does dmesg show any warnings that could be related to your hdd? Judging from the error message, your hdd is using a filesystem mounted using fuse (so probably ntfs) and halfway during the restore it just crashed/failed/disappeared. So you have to fix the problem with the hdd first before you will be able to restore your backup.