I think PC-BSD just doesn't like USB Flash drives.
For me, none of the 7.x versions have worked well in that regard, although under 7.0.x, the system would sometimes recognize the drive, but that was actually dangerous since writing to the flash wiped it clean (just my experience). The 7.1 releases have staunchly resisted to acknowledge me plugging in a flash drive; at least the data on it is safe.
However, I noticed when I installed 7.1 that the installer did see the drive which I had forgotten to take out.
Why the installed system is having these problems and how to possibly overcome the issue I don't know; in that regard I'm just as curious as you.