ZZZY,
I think I need to run a Linux tool to clean the file structure. Can I do that simply by rebooting, or do I need to know the name of this program and run it explicitly?

Running Chkdsk on the PC (as you describe) definitely did not work. Chkdsk ran very fast, found no problems and no lost files. When I ran chkdsk, there were four directories that had been corrupted; they could not be opened. Chkdsk made no change to them.

Thanks for your sussestion, though.
-toby