Thacy,
Thanks for suggesting "how to decrypt adopted storage". Gave me a nice trip down "the yellow brick road". I don't know if I have the ability (knowledge or equipment) to try the process that I found as the solution.
What seems weird to me is that although the card is not recognized as a USB device, I can access it with Airdroid and ES File Expolorer. I did so before and after taking the screenshot that I posted here, and it is on the card. I have done some searches. I found this:
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After the process is done, you’ll now be able to treat your MicroSD card as internal storage and use it for storing all kinds of data, and not just media. Thankfully, it’s not completely permanent. If you decide you’d like to use it just to store media, you can change it back by heading to Settings>Storage & USB, then selecting the card, hitting the top right menu and then select ‘Format as portable’.
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Maybe the reset removed that option, because I cannot select the card, and that top-right menu is not there.
Also, the process begins with a rooted device, which mine isn't, to get a 32-character hex key, but in Airdroid the first folder appears to be that key. The rest of the steps are on Linux, but the last step merely says "you can run cd /mnt/1 to browse the decrypted storage". I am guessing that merely gets access - not reformatting to make the card usable as portable, although I am also guessing that could be done in Linux too.
I was first thinking that in spite of the factory reset, the card is still usable as external storage. Maybe I should just be content with that if it is true. The fact that 16 GB is shown as internal storage, and that "Transfer data..." is greyed out would indicate otherwise. It seems to be treating the card as portable, with the screenshot stored there automatically.
However, I started this effort with the card because it was using so little of the 64 GB, so I thought I would swap it for a smaller card, and make better use of the 64 GB elsewhere (nothing to do with the factory reset - at least not intentionally - that was because of another problem).
In hindsight, I would have chosen a phone with 32 internal instead of my 16. As they say "Live and learn", and "a little knowledge is dangerous". Needless to say, I am probably more confused and in the dark than ever.
Update 2018-05-18
I am writing this in the hope that it will help someone else.
I found out that the card was not recognized in Windows because of the adapter that I was using for the USB slot. When I use a different adapter it was fine.
When I first removed the card from the phone, I powered it down first. Therefore, when I put it back in, as far as the phone was concerned nothing had changed. Removing the card and replacing it without powering down caused Android to give me the option of reformatting the card. However, the card does not seem to be totally okay. I plan to investigate further when I have time.