Are you using macOS Sierra? There's a known problem with the driver for the CH340 USB-serial chips used on the clone nanos and Sierra. You need to install a modified driver. More information on this thread:https://forum.arduino.cc/index.php?topic=428349.0I think this is the driver you want:http://www.mblock.cc/posts/run-makeblock-ch340-ch341-on-mac-os-sierraI can't understand why the manufacturer hasn't fixed this in the official driver yet. CH340 became by far the most popular chip after FTDIgate knocked the FT232 off its throne but I've noticed some new boards are using alternative chips, probably due to this issue.