COM fail when moving cable between two Arduino Unos.

Windows 10
IDE 1.6.11
Serial cable direct from host PC to Arduino
Two arduinos
#1 is a surface mount
#2 is a recently purchased old chip -style (dual inline chip)

Symptoms:
#1 programs fine on COM3
I unplug #1, COM3 disappears according to IDE.

I plug the same cable into UNO #2 (already powered up and running factory BLINK routine)
IDE hooks up as COM4 (??)

Then when I press the UPLOAD button for the identical program I get:

avrdude: ser_open(): can’t open device “\.\COM3”: The system cannot find the file specified.

Problem uploading to board. See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.

I have been through troubleshooting as suggested and I have read the forum for history. If there is any I can’t find it.

So… any clues as to what is going on?

thanks in advance…

Colin

OK. That was weird.

I restarted the IDE again (already done it once before to see if it worked)

Now it's OK.

But I don't know why.

So let's leave it there....

Thanks.

Windows can often tell the difference between boards, and will assign them different COM numbers. If the serial chip is different (ie, because one is a clone that uses a different serial interface chip) on the two boards, that'll definitely get them different COM ports.

Be sure check the tools -> Port menu to make sure you've got a port selected.

You must accept the fact that windows does its thing. There might be a way around this but I have not bothered to find it.

And hence you just select the other port in the IDE and happily continue ;)

It's important that windows assigns different COM ports based on device.

What a mess it would be if you dared to use a hub otherwise.

But who knows, I can plug in 4 different Nanos to the same USB port and not have to change ports in IDE. But port number changes when a different TYPE of board gets plugged in like an Uno or ESP8266 board.