the guys here have explained how the storing of pictures works and I agree with most of the comments.
I have had the bars as you describe ONLY EVER when the full file was not downloaded, copied or saved in total. Did you ever actually open the pics on your external drive previously and see the pictures as they should have been? If yes then I suspect that your external hard drive is dying and you should run a full diagnostic on it and maybe you can recover some of your pictures. If you never tested when you copied the files then it could also have been an error with your old system. Try coping all the files into directory and open it up with windows set to preview, watch and see if the complete picture is shown before the bars appear, or as the picture actually disappears. Another senario is that your external drive was almost full as you were copying the pictures, which used to corrupt some files, you could try copying from the command prompt a "FEW" files to see if this makes a difference.
Sorry for the vague reply.