I changed the verification post count from 2 to 1 so users only have to do the verification for their first post rather than first two posts. Sorry for the annoyance, spammers are a big problem with popular forums like this.
USB NICs in general aren't great, regardless of OS. The FreeBSD drivers for some of them tend to be pretty unreliable. This might just be that your USB NIC isn't well supported in FreeBSD, though I wouldn't expect the NIC to completely disappear in that case. Usually when there are driver issues, the NIC still shows up, but it just doesn't work right or at all. That might be a general USB problem with your system. One thing you might want to check is if you have the latest BIOS on the system, sometimes that fixes weird issues along these lines.
VLANs might be a good option instead.