This is a well discussed and easy to fix bug... Disconnect USB cable, go to Settings, Apps, and Show System Apps and find External Storage and Media Storage and clear data and cache on each one and reboot, then give it 5 min after full boot up to rebuild media databases and connect to USB and select MTP. Should be good to go.
EDIT: In some instances where the SD card contains a large number of (media) files, it can take more than 5 minutes for the databases to rebuild. There is no way to see if the status of this database buildup in the phone itself, so sometimes you just have to be patient.
Also note that some users have stated that this is not sufficient by itself, but sometimes the system cache partition needs to be wiped and rebuilt. Official instructions for performing this operation from Moto are here. Note that this operation can take significant time... 5-15 minutes, occasionally even longer, do not interrupt the operation.