I recently upgraded my Mac OS from Yosemite to Sierra and also upgraded my Arduino IDE to 1.6.12.
A new Serial port driver CH34x_Install_V1.3 for Sierra, was also installed and my laptop rebooted, now the serial port /dev/cu.wchusbserial1420 is visible and usable. I can upload code with no issues.
However, once I remove the USB Cable of the Arduino and later when I plug it back in - the serial port is NOT VISIBLE and the driver has to be reinstalled and laptop has to be rebooted.
(Meaning if I connect the Arduino USB cable evertime - I have to reinstall the driver, and reboot !! )
The driver I installed was from blog.sengotta.net/wp-content/uploads/2015/11/CH34x_Install_V1.3.zip
I have an Arduino Genuino and earlier when I had Yosemite with the appropriate driver, I never had this problem of reinstalling and this behavior is new with the newer version.
Was wondering if anyone has encountered this issue with the Mac OS Sierra .
Please see my attachments.