Can't save gnucash files

[Raven Long]

I'm running gnucash 2.6.11 on Windows 7. I run three separate businesses
off gnucash, and have for several years. Twice this week, on one of my
accounts, when it's tried to autosave, I've gotten the error message:

Could not make a backup of the file.

I've saved a new file in a temporary folder, then looked at the old files,
which are still there. Anything I try to do with them, I get the Windows
error message:

You require permission from the computer's administrator to make changes to
this file.

I AM the administrator.

I store the files in my Google Drive folder. Again, I've done this for
several years without problems.

Any suggestions? Why has this started happening? It doesn't seem to be
affecting my other two files.

Thanks,
Raven Long