After updating my iMAC to OS X El Capitan v. 10.11.1 and IDE to v. 1.6.5 I am unable to upload.
avrdude: stk500_recv(): programmer is not responding avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0x00 Problem uploading to board.
Tools: Board "Arduino Uno" Port /dev/cu.usbserial-A6008uLL Programmer "AVR ISP mk II "
RX LED on "FREEDUINO" (ver. 1.16 ) flashing.