Allow me to save all of you who follow this guide $14 and hours of headaches: don't waste your time with the internal USB port on the Soekris net6501. The little Sandisk Cruzer drives that fit inside the case are total crap. The two that I bought lasted less than a day each. I think that writing the 4GB PFsense image to them was enough to kill them. Unfortunately for me, it didn't kill them in an obvious way. In my case, strange things started happening in PFsense. DNS became half-broken, DHCP for new clients didn't function, etc. I finally realized what was wrong and threw the USB drives in the trash and bought some of these guys and the problem was solved:

http://amzn.com/B00ELQZD10