Sorry about cross posting. I posted here, but when no one originally answered, I had to try another site.

I reformatted only after trying all the advice given here and the other site.

If nothing else, reformatting has shown the problem to be with certain documents.

For instance, copying/pasting the contents of one doc into a blank doc and saving produced the same problem.

But copying/pasting one chunk at a time into a blank doc produced a file which now saves.

But for how long?

The original document is still not saving, and yet is, on the face of it, identical with the copy.