So the main processor seems OK, the few flashes are from the boot loader. What does your operating system think about the board? I'm not a Mac user so don't know how to check.
What is the chip closest to the USB port? A square one with legs at all sides or a rectangular one with legs at two sides. The former would be a 8U2 or 16U2, the latter a CH340 or FTDI232. For the 16U2, 8U2 and (probably) the FTDI, you can run the loopback test; see Loop-Back Test Instructions; the loopback test will fail with the CH340 so is not conclusive.