Large unknown (invisible) file on C Drive

Hi,

On a Windows 8 pro with WMC on C Drive (120GB SSD) windows reports 5 GB free BUT I have 50.6GB of unknown file

as reported by WinDirStat. 65 GB should be the correct size as per my usage.

I can't delete this unknown file as it does not show in file explorer or CMD prompt (dir listing)

Does anyone has this problem and how can I fix it ... I had this problem before, so did a clean install a week ago, my usage was about 65 GB and it has grown by itself ..

Many thanks

Sepp

Thanks Sepp