Raspberry Pi: Problems recognizing card when backing up with "SD Card Copier"

So this year I am using Raspberry Pis to teach a Fundamentals of Computer class.  I wanted to document a few problems I ran into.

I successfully backed up the Raspberry Pi the first time by using the new option under Accessories called "SD Card Copier".  But when I went to back it up again, it did not recognize the card!  I tried so many things to get it to work.  I thought it was the card reader, the SD card, and I tried formatting it, but that didn't work unless I used a specific program to reformat it.

Another colleague mentioned that it looked like it thought the cards were identical - and then I remember the option "new partition UUIDS".  When I formatted the card the first time, I did not check the box "new partition UUIDS".

So I formatted the backup SD card that wasn't being recognized with the SD Memory Card Formatter found here: https://www.sdcard.org/downloads/formatter_4/.  And then I ran the "SD Card Copier" again, this time selecting "new partition UUIDS" and it worked to back up the card again the next time!  Why couldn't I find this information anywhere?! :)