Thanks, but I had seen all of those links already.
I already did "dd" earlier, to completely clear the drive, so it wasn't a partitioning or GPT issue, or something left over from a previous installation.
There's no SATA or AHCI options to set in the "BIOS" of the Soekris. I think BSD is detecting the drives just fine, though, because they're all there, and the dd command completes without errors. It honestly looks like some kind of scripting bug when it tries to do the partitioning.
I found a workaround, though.
Run the entire system from an external USB stick! That's what I'm doing now. The 4g-i386-nanobsd image works fine.
Josh