I found another way actually. Let me know what you think.
1-In finder go to pictures then Photos Library.
2-right click and select "show package contents"
3-Select the "masters" folder as the parent folder
4-Open the parent folder and type Command + F. Select Kind = Document. That should display a list of all files in the folder and sub folders. (in my case "masters" would be the parent folder and not "documents" as in the screenshot)

5-You can then select all and drag them to a single designated folder on your external drive
NOTE: I am not sure what happens to files with the same name when i follow this method though...I believe they get renamed...