Connecting to the Sparkfun Redboard on Mac OS X Sierra
I'm using MacOS Sierra and an Arduino Pro Mini (from Sparkfun) with a breakout USB-to-UART board that uses an FTDI FT232 chip. I'm using the default Apple FTDI drivers (the ones that are bundled with Sierra) and I'm having no problems whatsoever.

Some people have reported problems using the manufacturer (FTDI) drivers that they had already installed after upgrading to Sierra. Some of them solved it by uninstalling the conflicting drivers and relying on the default Apple FTDI drivers uninstalling the conflicting drivers and relying on the default Apple FTDI drivers instead. It's worth a try.

OTOH, the CG340 driver doesn't currently work under Mac OS Sierra. Fortunately, Adrian Mihalko patched the driver and made it available at GitHub, along with some troubleshooting instructions.