Hi, I am sorry if there is somewhere else similar topic

I have 1 TB hard disc: 97.5 GB at C, and 833 at D. When I take a look at D drive - it says that I have 165 GB free on it, but when I enter the D drive and select all folders (included the hidden ones), it says that I have used 455 GB of that drive. So according to that I should have approximately 378 GB free, so I am wondering where are my 200 GB..? Could formatting the D drive help? Only I fear that I would lose all stored data on the drive, if I did something like that, so if that is the case, please, tell any possible solution.

Thanks in advance !