This is reported when using an Arduino clone with CH340 chips on board and "old" drivers.
Here:
it is explained how to remove the old river and a place to download a "Sierra compatible" driver.
I can't tell if that really works, but I read some comments of successful repairs. One guy said, he used a "better" USB cable, which solved the problem. Ok - I am still on Yosemite and wait to jump to Sierra only, when I find less reports on Sierra / Arduino crashes as I a have a mix of originals and clones.