Hello Smith,
As you told in your comment its a Linux vm. which acquired a disk space of "swap" partition with in it.which auto assigned while you create a default installation of it. and this can be the know "thief" which is stealing your 8 GIG of Disk space.
You can make sure it via running "df -h" command to see what is the Swap partition space is using.
All the best,
Please consider marking this answer "correct" or "helpful" if you found it useful.
Anjani Kumar
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VMware, Windows
Consulant
Email ID : Anjani.kumar@vmwareminds.com
Website : http://www.Vmwareminds.com