Debian places redmine files in non-standard locations to make it easier to build a redmine server that can handle multiple instances. However, this makes it hard to find things. Following are some locations we may need to find.

/usr/share/redmine/lib/plugins

/var/lib/redmine/default/files

I find this a pain for backups. I tend to back up directory trees, so you could back up /var/lib/redmine/*, but instead, I simply make that directory a symlink to someplace I already back up, like /home.

mkdir /home/redmine_data
mv /var/lib/redmine/default/files /home/redmine_data
ln -s /home/redmine_data/files /var/lib/redmine/default/files