Hey, so I've been working with a Uno R3 and have done 3 very simple beginner projects that have gone off without a hitch, but when I wired up the hardware for my 4th lesson and I went to upload the corresponding sketch I got an error message saying thusly:
{
avrdude: ser_open(): can't open device "\.\COM6": The system cannot find the file specified.
}
I'm pretty sure It's a legit Uno R3 clone and not a bootleg; the 500kΩ next to the USB data port is gold colored, the ATmega16U2 is a quad flat pack form factor surface mount IC and not a dual flat pack form factor. The board itself is marked with a 3rd party manufacturer's stamp of ELEG∞. When I plug the device into my PC's USB port the component 'L' on the PCB which is called out as LED_BUILTIN rapidly blinks 3 to 4 times (I'm too slow to tell!) in about 500 ms, then pauses for about 1000 ms, then stays solid. The LED labeled as 'ON' stays solid the entire time. However, once connected it fails to show up in the Windows 10 device manager and in the Arduino v1.8.13 software clicking 'Tools → Port' reveals that 'Port' is greyed out, however the status indicator in the lower right hand says 'Arduino Uno on COM6'. Putting the Uno into loop-back mode and selecting 'Tools → Serial Plotter' or 'Tools → Serial Monitor' both produce an error saying 'Board at COM6 is not available'. Wiring up the hardware corresponding to the last sketch successfully uploaded from Arduino 1.8.13 to the Uno reveals that the sketch is still loaded into the ATmega328's memory. Furthermore, while building this last project where I encountered trouble, after constructing the breadboard circuit, I connected it to the Uno board while it was still hot and looping my last sketch... my education didn't go farther than simple CMOS ICs and TTL devices that don't mind if you swap jumpers out of a pinout while powered, so I didn't unplug the USB cable because I'm a bonehead. I picked the board up while it was still hot so I could see the pinouts on the digital header better, and in the process I think I may have shorted the digital header to the unlabeled 6x header placed next to the digital header, or shorted all the way over to the USB connector's through holes. Just to cover all the troubleshooting steps, I rebooted my PC, I uninstalled and reinstalled the Arduino 1.8.13 software. I'm attaching an image of what I'm seeing in the Arduino 1.8.13 interface for completeness, although at this point I'm pretty sure the ATmega16U2 got fried.
Did your computer translate the "OO" of ELEGOO to ∞ ?
I agree with your diagnoses.
If you would drop a wire with 12V onto a Arduino board, then it is probably broken. If you short a few things, then it probably still works. I think you have bad luck. If the usb-serial chip is blown, then there is not much you can do.
With a programmer (A Arduino Uno can be a programmer or with a USBasp) it is possible to upload a sketch into the Arduino without using the usb-serial chip. It is also possible to use a programmer to set new firmware in the usb-serial chip (that does not help if it is broken). All those things are not worth the trouble. Grab a new Arduino board.
Try plugging it into a different usb port on your computer directly without an extender. If that doesent work check for corrosion on your computers ports. If that doesent work then it could be an issue with your board. I hade a uno clone once I accidentally gave 12 volts to its analog and now whenever I plug it in it gets really hot and starts smoking.
... 
So I switched USB ports and lo... there it is in Device Manager. I owe you a beer blakeblake! Whelp, now I know what that SMT electrolytic cap I sheared clean off of my PC's motherboard is filtering on. F**k me. I was sure that cap was attached to the PCIe x1 slot as it was right up against it, but looking at a view of the board, there's a whole mess of ICs that are close to that cap too. I really need to look in there and get the reference designation for that cap so Dell will tell me what it's original value was so I can replace it. I've got a rework station and a gazillion assorted components I never get to use anymore. It will be fun to do some level 1 rework. It never occurred to me to just switch ports as I use that same port to link my phone to my PC and sometimes use it for a backup keyboard while my usual mechanical keyboard dries out after a drink spill, and I've never had any trouble getting devices to connect or any I/O glitches when using it... I don't understand why one port would work but not it's neighbor, if I remember right they both come off of the same header on the motherboard.
Glad I could help! Sometimes when that happens to me some days one port would work an other days another would work.
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