You can use a tool like WinDirStat to identify what is using the space.
If you install the tool on your local workstation, you can then create a mapped drive to your server's F:\ drive and run the tool against the mapped drive.
Under Options - make sure you check the options to show free space and unknown.
If you find that you have a lot of unknown space being used, check to see if you have system restore enabled for that volume or shadow copies.
Jeffrey Williams
“We are all faced with a series of great opportunities brilliantly disguised as impossible situations.”
― Charles R. Swindoll
How to post questions to get better answers faster
Managing Transaction Logs