I created a new MFD by uploading an Excel .xls file (Create Synonym). The MFD created successfully and I was able to read the .xls data. I read the data and saved it back to the same .xls file. WF will no longer read the data. Same thing if I manually open the data in Excel, make any minor character change and save it. Seems the upload to create MFD creates a format that can be opened in Excel but that format does not tolerate being rewritten.