Thanks Con for the comments and advice.
Although I actually only have PD 6 , I am sure that is not the problem.
Indeed, these files were being extracted from an external hard drive where I had stored the images for safe preservation. I have also defragged recently.
I did not understand what you meant by "working directory". Do you mean that I should first bring these files back into my computer "documents", and have PD 6 access the files from there, rather than from the external hard drive? Doug