The solution – or at least temporary fix – appears to involve bypassing the “Automatic” location OS X assigns to the new connection when the USB Ethernet adapter is first plugged in.

Either the cable for USB Ethernet is not plugged in or the device at the other end is not responding.” The same dongles are generally found to be fully functional when later hooked up to a MacBook or MacBook Pro, suggesting the adapter itself isn’t at fault. Some reports claim the adapter functioned briefly when plugged into the right hand port, but then failed to work after that, meanwhile, giving only the error message “Cable unplugged.