Just in case it saves someone else some time, I was struggling to get a second serial port (Serial1) to work on a Mac Pro 2013 Trashcan under El Capitan using a Sparkfun FTDI 5.0v USB cable. I’d connect up Tx/Rx and GND on the Mega, but I could not get the Mac Pro/El Capitan to recognize the FTDI as a device and thus use CoolTerm as my debug port so that I could use “Serial0” (aka just Serial) for uploading the compiled sketches and for other purposes.
Power off the Mac Pro, pull out the AC cord and wait 15 seconds, thus resetting the System Management Controller. Put the AC cord back in and power up the machine. Now it recognizes the FTDI cable as an additional communications device (albeit with the catchy name of usb-serialAL0249AQ) and now I have to serial links to the Mega.
Wasted a couple of hours trying to figure out what was going on…It seems that El Capitan and/or the MacPro have a really uneasy relationship with USB…and it takes a power off, unplugging the AC cord to kick it in the naughty bits and make it realize you’re serious.
Oh, and I tried using a direct connection of the FTDI cable to the Mac Pro and via a power USB hub. It’s an industrial USB hub with a 10 Amp (yeah, 10 Amp) power supply. This is the USB hub: https://smile.amazon.com/gp/product/B015ZNWBYE/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1 : no frills, all business. Not a single spontaneous disk unmount since I started using it.