This is not the usual System Volume Information (SVI, for short) "missing" disk space.
I've been all over this system top to bottom, inside out, and there is over 200GB (that's right GIGAbytes).
Emptied Recycle Bin - there is only 1 user account, so it's not a case of several Recycle Bins for multiple users.
The space in all folders, including SVI, adds up to about 280GB of disk space. When viewing the disk properties from My Computer, it says there is 13GB remaining.
I've turned off Indexing (and I can see the contents of the SVI folder now) and I can see the shadow copy storage for System Restore points all add up to about 4GB total - which I've already accounted for in my 280GB total.
It seems that the disk summary is mis-reading something. I've seen the SVI with Indexing turned on add up to 10 or 15GB, but 200GB?
What am I missing?