2 Units, 2 Problems: Can't Upload! avrdude: verification error

While not new to Arduino, I am still a beginner. Last month I used one on 2 Halloween Props. I have 2 arduino Uno's both R3's, Made in Italy legit ARduino's probably 5 years old. I am a middle school teacher, so middle schooler have used these in the past. We don't currently teach Arduino in my classes.

Problems!

Uno #1: No port.
So I hook up the unit and the port is grayed out. So the computer does not recognize the port.

Uno #2: Computer sees the port just fine. I upload a sketch and Led #13, tx and rx blink a few times. Then the IDE says:
avrdude: verification error, first mismatch at byte 0x0000
0x0c != 0x62
avrdude: verification error; content mismatch
I have read this has to do with a bootleg or bootfile, I don't understand this.

Things I have done to both Uno's:

Changed USB cable
Used IDE 1.8.5
Uninstalled and reinstalled IDE 1.8.5
Uninstalled and installed IDE 1.0.5
Tried the web creator
Tried the web creator on a Mac (I have a PC)
Switched the computer chips between the Unos.
Tried all USB ports on Mac and PC
Reset PC
Read forums and internet for 3 hours!

Advice please. Thank you so much.

The flash memory of the 328 micro can be flashed 10,000 times. So that's a possible cause for the verification error in avrdude. And that's how far my knowledge goes.

For the non-recognised Arduino, what does device manager in Windows (or the Mac equivalent, I'm not a Mac user) say?

First one - if it's genuine, with 16u2, the first one is probably dead (damaged 16u2). It's pretty easy to do this, particularly as an inexperienced middle schooler. Oddly enough, the clones are more resistant to damage from abusing the power rails than the official ones.

sterretje:
The flash memory of the 328 micro can be flashed 10,000 times. So that's a possible cause for the verification error in avrdude. And that's how far my knowledge goes.

For the non-recognised Arduino, what does device manager in Windows (or the Mac equivalent, I'm not a Mac user) say?

Good to know there is a life span. If the kids flashed them 20 times a class daily for a few weeks over a few years that could be it.

Device manager does not even see it. The Arduino powers up but does not show up anywhere on the computer.