I have 2 Arduino boards. One is working fine but the other has a flashing light next to 'L' and I can't select the correct serial port (/dev/tty.usbmodemfd131) because it doesn't show up in the list and therefore can't upload any sketches.

Can anyone help?

Looks like your DFU chip went stupid. Try this: http://arduino.cc/en/Hacking/DFUProgramming8U2

It hasn't been updated for the R3 boards, but all the firmware is in your Arduino IDE: arduino-1.0.1\hardware\arduino\firmwares

Skip the 10k resistor if you have an R2 or R3 board (haven't found a board that needed it yet).

Many thanks, I'll give it a go.

I have tried the above but when I type in 'sudo dfu-programmer at90usb82 erase' to the terminal window the response is 'no device present'

Also nothing seems to change when I short out the 2 reset pins.

Could the board be damaged in some way?

Do you have an Uno R2 or R3? If it is an R3 you are going to use:

sudo dfu-programmer at90usb162 erase

The tutorial needs updating, sorry about that.

I am getting the following when trying - cd Downloads/

cd Downloads/ -bash: cd: Downloads/: No such file or directory

I'm using MacOS 10.8.2

Hmmm... how comfortable are you with AVRDUDE? Do you have a programmer or female by male jumpers (6)?

ah...my knowledge of these sort of things is limited. I'm sure I can rig up a male-female lead/jumper though and am willing to have a go.

Most of what I have learnt is through forums.

Excellent. What we need is to hook that 3x2 header up next to the USB. They are numbered different than chips:

(MISO)1 - 2 (Vcc) (SCK) 3 - 4 (MOSI) (RST) 5 - 6 (Gnd)

...where the little silk screen dot indicates Pin 1. For reference, you were shorting Pins 5&6 to reset the Atmega8u2/16u2.

Upload Nick's programmer to your working Uno: http://gammon.com.au/Arduino/Atmega_Board_Programmer.zip

This will be the programmer. It will verify the connections by identifying the chip. We will have to switch to Arduino as ISP when we get to the AVRDUDE stuff.

Anyhow, hook Pin 1 of the Target to D12 of the Programmer Pin 2 to 5v, Pin 3 to D13, Pin 4 to D11, Pin 5 to D10 and Pin 6 to Gnd.

Plug the USB into the Programmer Uno, open the serial monitor and select 115200 baud.

Copy and paste what you get here.

OK so i've connected up the 2 boards, set the first board as a programmer and set the baud to 115200

The result was:

Atmega chip programmer. Written by Nick Gammon.

That is what I like about Nick's program, it tells you when you are hooked up correctly...

Could you grab a snapshot of you setup?

I’m hoping I’m not being stupid but I guess you need a photo!?

I really like your female connectors... and you are indeed hooked up correctly.

Anyhow, same response? Crap, crap, crap. I am afraid that 16u2 is shot (Uno R3 USB2TTL comms). You will need to remove that 16u2 and replace it in order to recover the board. Sorry about that. The only other thing I can think of is a last ditch test of finding a MS Windoze machine and installing as normal (if it will try) and tapping the Pin5&6 for reset again to see if there is a response.

Yes female connectors of a sort!!

Thank you for your help and your time, I will have ago at replacing the 16u2, after all if it’s already broken I can’t do anymore damage!

Yeah, a partially broken board if the best place to "play" with this sort of thing. Unfortunately, the TQFP package is a major PIA. I haven't attempted it yet, but I suspect a SMD rework heat gun, insolated gloves and tweezers will be involved.

I heard of many 16u2 failures on the Mega2560 and always wondered why the Unos were unaffected. Not the case apparently.

Good luck.