My TB/Ethernet adapter is not detected by my rMBP

My rMBP has two thunderbolt ports, one of which has a VGA adapter plugged in (works fine, in both ports), and the other has an Apple Thunderbolt to Gigabit Ethernet adapter. This is the second such adapter I've tried, since the first was pre-owned and suffered the same problem as this one, so was returned on the assumption that it was broken.

Essentially, the adapter is just not detected by the MBP at all. No corresponding device in Network Settings, System Report oddly reports both TB ports as "Not Connected".

I know the ports are at least sort of functioning since the second display works fine in either one, and it's unlikely that the adapter is broken since it's the second one I've tried. Googling brings up lots of suggestions along the lines of disabling IPv6 or renewing a DHCP lease, but I can't even get that far in the process since the adapter itself isn't available for configuration.

I'm running 10.10.2 - can anyone suggest what might be wrong?