If you go into System Preferences > Accounts, you can right-click on your user and choose Advanced Options. I copied my home dir to the HDD, then linked it here

If you go into System Preferences > Accounts, you can right-click on your user and choose Advanced Options. I copied my home dir to the HDD, then linked it here. You get more nuance from symlinks, etc, but this was a simple method which worked for me. Other people mentioned symlinking some dirs, but no one had mentioned that you could move your entire user directory this way.