Where did my free space go

I was cleaning up my HDD that had 50 GB of free space and 50GB used. A couple of the files were around 7 GB in size.

Defraggler ran for a few hours, and now reports that I am down to 25GB of free space. What the heck happened? How can I reclaim the free space that I had originally?

Thanks for any help!

Have you tried doing a full restart of your machine ?

Yes, thank you. After restart it reports 58.3 GB used and 39.7 GB free. So at least 8-10 GB has gone missing. I really have no idea what to look at next.

Are you using Windows 10 and recently updated to version 1903?

If so then Windows itself may have 'reserved' that 8-10 GB so it's no longer available as free space, see this article:

https://www.ghacks.net/2019/01/08/windows-10-reserved-storage-explained/

- Run Antibody's Wiztree. See how large the size of the folder "System Volume Information" is and what the dates are of the sub-folders in that folder. When were these folders created ? When you ran Defraggler ?