user13937: I have an arduino uno Rs board and when connected to USB the board is dead. Yet when connected to the external supply the board will come to live. Power LED on and led near pin 13 blink on and off all time. I tried another computer and other new usb lead. Still computer will not see the board. When usb lead is on checked the 5 volt on the board and it is ok. Even checked the usb lead with multi meter and all  four connection are ok 
Can you give me some advice

1.Yes I bought the new board from different source but still waiting for it to arrive.
2.  My clone board Arduino uno using microcontroller Atmega329P-PU and interphase chip Atmega16U2]
3 but the video link is still of help as my new board is as the one shown in the video. Will use it late. Great!!!
There should be a similar explanation for the one I have in the picture

user13678: Does the Arduino show up at all in device manager or in mac/linux.(not sure what you are using)Is your board authentic or a clone? Did this happen the first time you plugged the arduino in or after uploading sketches etc? I had screwed up my firmware for my board that acted similar to what you describe.(I was trying to flash a different firmware) Eventually i ended up reflashing the right firmware and it went back to working again.Maybe it's that, maybe it isn't.

user3638: I had a similar problem. though it was easily solved but not obvious.
I couldn't connect my nano to the computer, but other devices worked fine. After a while other devices started to be faulty such as portable hard-drives and my CC3D flight controllers. After a cable replacement all the items started working again even the Arduino that i otherwise had discarded as broken.
It might not be the solution you are looking for but im leaving it here in case others comes for a solution.

13937: <p>I have an arduino uno Rs board and when connected to USB the board is dead. Yet when connected to the external supply the board will come to live. Power LED on and led near pin 13 blink on and off all time. I tried another computer and other new usb lead. Still computer will not see the board. When usb lead is on checked the 5 volt on the board and it is ok. Even checked the usb lead with multi meter and all  four connection are ok 
Can you give me some advice</p>

<hr>

<p>1.Yes I bought the new board from different source but still waiting for it to arrive.
2.  My clone board Arduino uno using microcontroller Atmega329P-PU and interphase chip Atmega16U2]
3 but the video link is still of help as my new board is as the one shown in the video. Will use it late. Great!!!
There should be a similar explanation for the one I have in the picture<a href="https://i.stack.imgur.com/ngtdr.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ngtdr.jpg" alt="Uno board as descripted"></a></p>
 13678: <p>Does the Arduino show up at all in device manager or in mac/linux.(not sure what you are using)Is your board authentic or a clone? Did this happen the first time you plugged the arduino in or after uploading sketches etc? I had screwed up my firmware for my board that acted similar to what you describe.(I was trying to flash a different firmware) Eventually i ended up reflashing the right firmware and it went back to working again.Maybe it's that, maybe it isn't.</p>
 3638: <p>I had a similar problem. though it was easily solved but not obvious.</p>

<p>I couldn't connect my nano to the computer, but other devices worked fine. After a while other devices started to be faulty such as portable hard-drives and my CC3D flight controllers. After a cable replacement all the items started working again even the Arduino that i otherwise had discarded as broken.</p>

<p>It might not be the solution you are looking for but im leaving it here in case others comes for a solution.</p>
