What is on my drive that is taking up so much space?

My Passport 500 Gigs (465 Gigs) shows 265 Gigs free of 465 Gigs. The total of all files I have put on the device is 136 Gigs so I should have 329 Gigs free not 265. I understand that the software puts files on automatically so is it possible I have things on there twice?
How can I delete what the WD software has placed on the drive and just use it manually as a storage device?

The only WD software that will put files on drives is WD Smartware and WD Backup, and only then if the drive is connected to the computer with them installed and it’s actively told to do a backup. In the case of Smartware, they can be found in the WDSmartware.stor folder in the root of the drive (not sure about WD Backup as I don’t use it).

More likely is that you’ve got files either in a hidden folder or the recycle bin (if in Windows) stored on the drive. View the file/folder tree using either Windows Explorer, Finder or whatever the appropriate file manager program for your OS is, and check that it’s set to show hidden files/folders. Also if you’re in Windows, try emptying your recycle bin with the drive connected.