OK, I had a whole bunch of spurious network adapters stuck in my macOS Sierra network preferences, so I deleted them expected that USB discover would bring them back. Well, I then got a strange error message:
Would you like <Insert name of adapter> to return next time it is connected to your Mac?
Not understanding what this meant, I just said, NO.
Well what it means is that the next time you say plug that thing in, it will not show up on your Mac, so I don’t know when you would ever say yes to this.
The solution to this is pretty message and it involves blowing away a plist (property list). Specifically you need to start terminal and do a:
sudo rm /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist