Try exporting the image from the container field before attempting to open the file in Photshop (opening the exported copy, not the one from the external storage location). Does that work?
I suspect that your files were originally inserted with the "by reference" option. The files that you insert into a container field with external storage should not be inserted "by reference"--so that may be the source of your trouble here.
You may need to set up a script that exports the field contents of a container field and then re-inserts the exported copy of the file, but without the "store a reference" option specified.
I initially inserted the images into the container fields via drag n' drop method, is that "by reference"?
Exporting from the field before opening in Photoshop does work.
Would the script you describe work on the whole DB, at once, or just one record at a time?