There shouldn't be any hidden folder on an external drive. There may be some hidden files but those are system files placed there by the OS and really don't need to be seen as you can't and shouldn't do anything with them. If you delete them the system will just recreate them.
But to show all files on every drive connected in any way to your Mac open terminal and type this in and hit the enter key.
defaults write com.apple.Finder AppleShowAllFiles YES
To stop showing all just change the YES to NO