This is more a note to myself (so I won't waste time next time I break the networks settings and have to reconfigure everything) rather than a legitimate blog post, but here goes.
It is impossible to use the transmission-daemon to download stuff onto a mounted hard drive when running as the default "debian-transmission" user. Permission errors abound. It's much easier to change the user that runs the transmission-daemon daemon.
sudo vi /etc/init,d/transmission-daemon and change the user to "pi"
sudo chown pi:pi /var/lib/transmission-daemon/info/settings.json
Much less headache.