Difference between MY COMPUTER disk usage and actual FOLDER SIZES?

I have a local C: drive with 976G capacity showing 275GB free space, meaning I should have approx. 700GB of files on the drive.

However, when I show hidden folders and check the 'Size in Disk' sizes on all folders, including the Root files I get a total of less than 440GB.

Any ideas what might be stealing the extra capacity?

I have Symantec System Recovery 2013 on this PC and note that a full image file, with compression, is 694GB.

Grateful for help.
Who is Participating?
David Johnson, CD, MVPConnect With a Mentor OwnerCommented:
You need to run these tools as an administrator to get accurate results

System Volume Information showing as 0  tipped me off.

Left (right click run as administrator), Right (just double click)2013-06-29-1410.png
David Johnson, CD, MVPOwnerCommented:
System Volume Information is taking up the remaining space
Config.msi is another item that may not be shown

You have to uncheck hide protected operating system files
Haresh NikumbhSr. Tech leadCommented:
check it out with Disktective freeware tool which will trace used disk on the system

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gilbey666Author Commented:
Have run disktective and attach the results. The D: partition results match but the C: ones show up the anomaly.

Have unhidden protected operating system files but this does not alter results, I see swap file, hibernate etc. Can't locate config.msi
Haresh NikumbhSr. Tech leadCommented:
try foldersreport tool which is freeware too
gilbey666Author Commented:
after scanning with foldersreport still same results.

only thing I can think, we back up to a Drobo and I had that drive shared and mounted on this PC with the files available offline. The Drobo was reset recently with new shares, different ones, that would account roughly for the difference.
you can also run chkdsk - it often helps in these cases
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