I turned the computer off and left it off all night and today. Took the thumb drive with the library out before starting up. Only program running was Chrome (downloading the newest Calibre. I don't use dropbox or anything like it, don't share the library with any other programs or users. It just lives on the thumb drive.
Opened the program, asked to use the library, says corrupted, goes through the 3 steps again...and comes back with the same error message.