reply to my own post :
pretty sure the ATMEGA16U2 has lost its programming ..
what hardware is needed to reflash it ?
-- was able to dig up the info / for the record
A) avrdude.exe / avrdude.config is not located where the doc say -- it is in:
B) the hexfile is buried pretty deep
C) I used a Nano as a AVRISP which has the same pinouts as the UNO descibed in http://arduino.cc/en/Hacking/Upgrading16U2Due
D) the Nano would error out ("avrdude: stk500_getsync(): not in sync: resp=0x15") without the required a 120ohm resistor between Vcc & reset http://laclefyoshi.blogspot.ca/2011/01/note-of-caution-for-arduino-isp.html
--- this was all to no avail - my Due still can not be programed via the USB Programming port
to program my Due requires a tenitive connection to the USB native port which resets after every upload (^#&$%&%!!!)