unable to export original files

Hello-

When attempting to export original files that have been loaded in the past I recieve a pop messages stating "Some images could not be exported at the size you requested because we couldn't access the full size original(1). I haven't accessed these files for awhile, possibly months. I have no issues exporting original sized files from recently loaded picture files. all my pictures are accessed from external hard drives. Why is this occuring, how can I fix this and prevent from happening in future? thank you. 

Peter