Metadata error message

Thanks Chuck.  I get the same error message if I try to open a different group of *.nef files than the ones I have been working on, even if the computer has been sitting idle for a length of time.  Your reply got me to thinking though.  After opening a group of *.nef files in PS CC from ACR, then saving the images in another format and Closing All files in PS, I get a message asking me if I want to Save Changes.  I have been clicking NO.  This is puzzling since the files were just saved.  I assumed the changes to the metafiles were saved when uploading the *.nef files from ACR to PS (and it appears that they are).  However, I will start clicking YES to the Save Changes message in PS and see how that affects the Metadata write operations pending warning message.