Thank you everyone for your help. I finally solved the problem of Joplin continually attempting to sync a 12M file that I had removed from the Joplin note . I also changed on and off the selective sync, several times in DropBox, the apps folder in Joplin. Nothing seemed to help. So in Windows File Explorer I searched for anything that had Joplin in the name and deleted it, except the downloaded file to install Joplin, and the exported notes file. Reinstalled Joplin on the problem PC. No longer trying to sync the 12M file in a note. In Joplin I probably had encryption on.
But now I'm getting the dreaded " One or more master keys need a password. Set the password" banner at the top of the Joplin app on every device (PC, laptop, android, chromebook), but the sync process on all machines is now quick. I've searched the web for this message, and found a few matches, none were a good solution. Plus, on the Windows PC that had the 12M file issue, after making changes to notes on the other devices, when it syncs to the Windows PC, it creates a 'Conflicts' folder in Joplin, but the updated notes are in the 'real' folder, and in Conflicts they are the old notes.
In Joplin I've looked at the keys and I have at least two IDs, except on the Windows PC . The link "One or more master keys need a password. Set the password" doesn't give much information, and this link is long, with several entries describing a long and problematic process that may or may not resolve the problem.
I suspect I've the 'Set the password' problem by deleting all the Joplin files I could find on the Windows PC, plus deleting, more than once, the files in the DropBox / Apps / Joplin folder.
Think my next step will be to decrypt on every device and do syncs. Is there anything else I can do, or another solution to my problem of (1) the 'set the password' banner , and (2) the Windows PC getting the Conflicts folder when I sync changes that were made on other devices.