It didn't change to dynamic by itself. I had a need to expand one partition and shrink another. When I did that, I got a message that it would be changed from basic to dynamic. The message said that a dynamic disk could not be a boot disk, but, since it was NOT a boot disk, I allowed it to make that change.

I'm not sure that it being a dynamic disk was the cause of the problem of not seeing it in File Explorer. I think that since I had a newly installed ssd with windows 10, when I reconnected the 2Tb second HD, the system didn't know what it was, and tagged it as "foreign." I believe it was that "foreign" tag that was the problem.

Thanks again.