I usually keep quite a close eye on my disk space, but recently I was down to about 70gb free on a 512gb Surface Book instead of the usual 150gb.

I didn't remember creating any enormous files, so I checked things out with the excellent Windirstat. 

This reported a used disc space of 320gb whereas properties of the drive reported almost 390gb used. I ran it again as administrator - still the same.

I did a bit of searching and came across FolderSize - running this as administrator showed the folder System Volume Information in the root of the C:\ drive, consuming 70gb+ of data. 

Eureka!

A bit of further searching, including my memory in this search, a light came on - System Restore points! Control Panel>System>System Protection>Configure showed me:-

There was my disc space - I don't remember setting it to a max usage of 100% - but something did. I usually run with a MAXIMUM of 10% for System Restore. 

I checked the restore points, and only saw 2 listed. I deleted the restore points, and set the max usage to:-

And I immediately saw my disc space back.

I will keep an eye on things and may well increase the max usage.

Hope this may help solve someone elses missing disc space.