I have upgraded my Macbook Air CTO 8Gb ram (mid 2015) to El Capitan. After that the USB ethernet adapter don't working... first always said cable unplugged, after i removed the interface i can't add back. I also tried to reinstall the moschip usb driver and edit the /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist but no luck.
Finally I find this in console:
01/10/15 14:40:12,461 com.apple.kextd: ERROR: invalid signature for com.moschip.driver.moschipUsbEthernet, will not load
I googled it and run:
sudo nvram boot-args="kext-dev-mode=1 rootless=0"
then booted to recovery mode (hold command + R at boot)
and in terminal run
reboot and then driver loads again... huhh :)