See if this is any help click here.
Treesize click here may tell you where the missing space is. If it doesn't account for the missing space, try defragmenting your drive.
Not sure if relevant or not but, also check how much space System Restore (SR) is taking up for your external drive if enabled - by default SR occupies 12%. Just checked mine (the System Volume Information folder in C:) & it's taking up >25%, though it says 12% in the settings! G