Installing pfSense onto a SSD equipped with a USB Aux Port.
I was pleasantly surprised that the GSkill 64GB SSD drive (FM-25S2S-64GB) I chose to use with a Soekris net5501 was recognized as a USB drive by the pfSense installer (option 99).
While I powered the drive via the Soekris I attached its USB 2.0 port to my desktop machine and then booted the pfSense CD.
After the installer completed I connected the drive to the SATA interface on the Soekris and powered it back up.
The boot and swap partitions were not initially found but a simple change to the fstab entries (listed below) fixed that. OK, I made that sound a lot less frustrating than it actually was!
$ more /etc/fstab
/dev/ad1s1a / ufs rw 1 1
/dev/ad1s1b none swap sw 0 0
After that it booted without any problem and everything has been running fine ever since.