I've used Sainsmart relay boards with MR3020's in the past without issue but now I've run across a show-stopper.  I think that the latest version of the MR3020 (V1.9) has changed. During reboot the relays on the Sainsmart start cycling very quickly. I think this is due to gpio ports associated with the USB port are floating or  in a wrong state. This cycling eventually damages the USB port on the MR3020. I've bricked one MR3020 and would prefer not to kill another. Can anyone think of a work around?