Just for some extra clarity.
I have un-checked hide protected system operating files.
There are two system folders Recycle Bin and System Volume Information.
Both of them come to a total of about 100kb. So they are not contributing to the disk usage at all.
You probably have a lot of old volume shadow data taking up capacity. You can do like this to reclaim it: Reclaim or Resize disk space from "system volume information" by Britec - YouTube
It's stored in the System Volume Information folder, but the OS by design prevents you from enumerating the contents. The commands mentioned in the video can be used to manipulate how much capacity is used by shadow storage.
Minor edit, fixed typos
Thanks for the response. I think you are right about the system volume information.
Last night i formatted the entire drive and copied all the files back onto it. It now displays the correct space usage on the disk.
I will give that video a go. Thanks again.