Hi Harry,
My name is Igor, it's a pleasure for me to help others and I'll do all my best to help you.
Please run command prompt as administrator, run
chkdsk D:
command and show the result. This is the most exact result.
Please note, some system related files may be inaccessible for a programs you are running, so these files will not be counted. It is usual enough the Treesize shows only a part of used space.