So I got my Nexus 7 2013 tablet with marshmallow to work once.. I see and issue with everytime you plug in a device you have to tell it which mode. I used file transfer and it worked for the shield and tuner.. I accidentally turned off USB debugging.. lost all comms.. claimed the USB to Arduino was good but was not.
Nothing worked to get the units communicating.. I did not know if this android version is set up to work with the shield