What are "Additional Files" and How Do I Clear Them?

My 2Tb Passport drive has 968 Gb of “Additional Files”. I checked the documentation and the description is: “These are the system files and hidden files that your computer’s operating system put there when you installed the device.”
The files don’t show up anywhere and I can’t see what they are or any way to clear them. They are clogging up my backup drive and restricting my backups.
Any ideas how to see/clear these files without having to reformat?

Hi FatFreddiesCat,

Have you tried to delete those files manually?

Morever, It would be best if you contact WD Technical Support for the same.

To Contact WD for Technical Support
https://support.wdc.com/support/case.aspx?lang=en
The link below will allow you to call support.