user1666: This is the first time I'm using my Arduino. It's an Arduino UNO R3. I have two boards. One works, but the other doesn't. I changed the ATmega328P from the broken one to the good one and it works, so I can confirm that the controller works. The issue seems to be in the board. 
When I plug the board it doesn't come up either in Mac or in Windows. The LED blinks and the output pins have voltage, but it seems that it's only getting electricity from the computer, it's like the data channels don't respond. The other board works perfectly both on Windows and Mac.
I checked the logs on the Mac and nothing changes when I plug the board. It's like I'm not plugging anything.
Is there a way to reset the board only? Maybe a component was badly soldered in the factory. I can't use the warranty because I got the board last year and never used it until now.

user10794: I have a sketch, that runs on a Uno, that detects chip signatures on other boards. You can use that to verify if the other board is good.
http://www.gammon.com.au/forum/?id=11633
You just need six jumper cables, like this:

From your description it sounds more like the Atmega16U2 is not working, that is the USB interface chip, the small chip near the USB socket. You can use the same sketch and a similar interface to check that as well.

That should at least show that the chip(s) are alive and giving valid signatures. 
You could use About this Mac -> System Information to see if the problem Uno is listed as some sort of USB device.

user60: The R3 uses an ATmega16u2 in place of the FTDI. It is possible that it is not loaded, busted crystal, or it has no local power to it. It is a small SMT.
You could use your good Arduino setup as ICSP programmer to talk to the ICSP of the 16u2 to see it is alive. And if so then program it.

1666: <p>This is the first time I'm using my Arduino. It's an Arduino UNO R3. I have two boards. One works, but the other doesn't. I changed the ATmega328P from the broken one to the good one and it works, so I can confirm that the controller works. The issue seems to be in the board. </p>

<p>When I plug the board it doesn't come up either in Mac or in Windows. The LED blinks and the output pins have voltage, but it seems that it's only getting electricity from the computer, it's like the data channels don't respond. The other board works perfectly both on Windows and Mac.</p>

<p>I checked the logs on the Mac and nothing changes when I plug the board. It's like I'm not plugging anything.</p>

<p>Is there a way to reset the board only? Maybe a component was badly soldered in the factory. I can't use the warranty because I got the board last year and never used it until now.</p>
 10794: <p>I have a sketch, that runs on a Uno, that detects chip signatures on other boards. You can use that to verify if the other board is good.</p>

<p><a href="http://www.gammon.com.au/forum/?id=11633" rel="nofollow noreferrer">http://www.gammon.com.au/forum/?id=11633</a></p>

<p>You just need six jumper cables, like this:</p>

<p><img src="https://i.stack.imgur.com/8TLeK.jpg" alt="enter image description here"></p>

<p>From your description it sounds more like the Atmega16U2 is not working, that is the USB interface chip, the small chip near the USB socket. You can use the same sketch and a similar interface to check that as well.</p>

<p><img src="https://i.stack.imgur.com/MfPZf.jpg" alt="enter image description here"></p>

<p>That should at least show that the chip(s) are alive and giving valid signatures. </p>

<p>You could use About this Mac -> System Information to see if the problem Uno is listed as some sort of USB device.</p>
 60: <p>The R3 uses an ATmega16u2 in place of the FTDI. It is possible that it is not loaded, busted crystal, or it has no local power to it. It is a small SMT.</p>

<p>You could use your good Arduino setup as ICSP programmer to talk to the ICSP of the 16u2 to see it is alive. And if so then program it.</p>
