This must be done from userspace. I'm not sure what debian uses for init system but Nemo is using systemd which does this by itself. You'll have /var/log/journal/randomname dir with logs which are not plain text, and you'll have to use journalctl --directory=/mountpount/var/log/journal/randomname to see them. But that happens rather late, so watchdogs are first to solve (and there is nothing in dmesg if they kick in - it's just like pulling the PSU plug)