I am having the nearly exact same issue. Just got a new SkyTech computer last night. It came with this same drive installed. I have 162GB of unaccounted space. I've done all the same things, WinDirStat run as admin, Disk management, Partition Wizard, all are telling me I have about 90GB used but File explorer and Partition Wizard space analyzer both say I have 251GB used. Maybe bad drive? Bad set up? How could a new drive already have 160+GB of space already encrypted or locked up as System resources?