I can't see what's on my External Storage disk

My startup disk is full. So, I've been trying to copy all of my iTunes stuff to an external storage disk. It's a My Passport that I purchased several years ago and have moved photos to in the past.

My problem is that while I can see the external storage in finder, when I click on it there are no folders or any files on it that I can see. So, I'm confused. Because I also receive an error stating that my storage is full and that iTunes cannot be copied to it. Help.