OK, test results:
I mounted the HD, opened an album folder in Finder, and duplicated one track file successfully. Next, I launched MC 20.0.124 and did an import-single-folder (with all media types checked for import) of that same album. It showed up in the library, including the duplicate track. I tried playing a track and it definitely played through the laptop speakers.
So I tried typing a value in a blank tag field for the first track, and it "took" in the library. So then I tried the update-tags-from-library command and got the error message instantly. I also tried selecting the album icon in album view and executing the same command, as well as selecting all individual tracks in detail view and executing it, and got the error message instantly in all cases (with the appropriate number of tracks reported in each case). So I deleted the album, and then opened the "Removed Database" playlist and manually removed all the tracks so they could be re-imported. I tried re-importing. The first time I did so, I thought the new test tag value had come over, but when I repeated the process, it didn't, so I must have failed to quit MC the first time prior to re-importing.
I repeated the same import, test tag, try to update tags from library, delete, clear Removed Database, quit MC series of steps several times and got the same error message and the test tag would not re-import.
I also successfully deleted the duplicate file track on the HD via the Finder.
I checked my version of the Paragon drivers and they are up to date and rated for both Mavericks and Yosemite, though I had done one incremental OS update since my last tag editing session using MC 19.x (I'm on OS 10.9.5). But I think the above test shows I can successfully write to the drive. I haven't ever had to adjust antivirus or firewall settings in the past to do this, and I haven't changed anything there since the MC upgrade.
So I think we've eliminated a read/write issue to the HD file system.
I don't think I've added any applications to this laptop since my last tag editing session with MC 19.x. Splashtop Streamer runs in the background, but I haven't been connecting remotely to this machine, and that also was present under MC 19.x.
So we're back either to some MC setting I have forgotten or set wrongly, or unexpected behavior in the app, or mystery factor X.
Thanks for any light you can shed. Has anyone else reported similar problems? If not, I'm inclined to think I have some setting wrong . . . .