Almost half of disk space unaccounted for on Windows 7 computer

I'm trying to determine what's eating almost half of disk on my Windows 7 Lenovo laptop but haven't been able to determine what. I read some of the answers on this site but none of the causes seems to be the cause of my problem.

What I've done so far:

• Checked Windows' System Restore points but only minimum use, either way I deleted around 2GB if usage from there
• Used WinDirStat and it came back with a total of 71.3GB usage at the root level
• C Drives properties says 135GB being used out of 138 GB total, ie, 135 GB - 71.3GB = 63.7GB missing somewhere
• No "Unknown" files reported by WinDirStat
• Windows Disk Management utility doesn't report any unused partitions or unclaimed space
• chkdsk comes out clean
• Windows disk cleanup tool doesn't report anything big, just small stuff in the lower MB range
• Lenovo backup tool hasn't been used nor there is a folder for the backup images

Any idea on what to look at and/or do?

EDIT: some additional info...

Well, I found what the problem was....

I installed SpaceSniffer and initially it gave me the same numbers as WinDirStat, but it also gave me some warnings about some directories it could not read, and among one of them was the Lenovo RRbackups backup directory that I thought it was not there since neither Windows Explorer nor WinDirStat showed.

So I ran the tool again but with Administrator privileges and voila, it found the big a... GB file inside the RRbackups directory that the Lenovo utility creates and that I couldn't see before. It turns out somebody ran the backup tool without me knowing.

I should have suspected that it was a permissions issue even though I was logged in with an account that is a member of the administrators group. Way to go Microsoft with Windows 7, an administrators group that doesn't have administrators privileges.... go figure...

Thanks all for the tips...

Run WinDirStat in administrator mode (it's a right click option). Some files can't be seen without this.

My culprit was MSSQL log files. One of them was 170GB.

What works for me was lowering space for Previous versions of files (http://windows.microsoft.com/en-us/windows/previous-versions-files-faq#1TC=windows-7).

1. Click Start Icon
2. Left Click on "Computer" and click on Properties
3. Click on "System Protection" on left side
4. Click on Disk and "Configure"
5. Lower your quota

I had a similar situation on a Windows 7 laptop where nearly 500 GB was "missing" on my system partition. The partition was full, but Windows Explorer was unable to detect the missing space, and the "windirstat" program simply showed a 500 GB "unknown" entry.

However, the TreeSize Free (Administrator) program (https://www.jam-software.com/treesize_free/) was able to show what the problem was, namely that someone had misconfigured the Cobian Backup program such that the source and destination directories were the same. As a consequence, it recursed until the partition filled up AND made nearly infinitely long directory paths that neither Windows Explorer nor windirstat were able to handle.

Perhaps needless to say, none of the Windows directory removal tools, including cmd rmdir, would work to remove this mess and it was with great difficulty that I finally managed to clean it up.