What is “<Unknown>”?

WDS shows a certain item labelled “<Unknown>” and recently someone posted a comment asking for details:

i think it would be nice to put in the faq or in help or in some docs what’s in the <unknown> space

sometimes it’s an huge amount of space! like 700 or more MB ! i think it could be the “system volume information” but! what mysterous things are within this space? he he he!

This mysterious item is just the difference between what Windows reports as the free space on the volume minus size of the files WDS can access. Please note the part WDS can access! This is the important point here. WDS cannot access the files under System Volume Information on all the (NTFS?) drives, so it cannot sum up the sizes of these items. And by the way, we have had reports of up to 30 GB of “<Unknown>” space.

Now what is stored in this directory? If you could gain SYSTEM access (and I will not tell you how that is possible …), you would see that the system keeps some binary log files there, but these take up only a small portion of what is stored in there. The biggest portion is usually being taken up by the System Restore Points (SRPs) you can create (or that are automatically created by software installers). The contents are a dump of the registry at the time the SRP was taken and of relevant files (usually from the system folders). I personally turned this feature of since I have a different backup strategy – however, this may not apply to everyone, so you’ll have to live with the “<Unknown>” item :mrgreen:.

There also have been reports that some third-party software stores data there, but I cannot confirm this. Also in the very unlikely case that your file system is corrupt there may be a non-zero sized “<Unknown>” item. In that latter case run a file system check and the problems should be fixed afterwards.

// Oliver

PS: If you run under a non-privileged account this could also cause a lot of files to be inaccessible and therefore to count as “<Unknown>” …

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99 Responses to What is “<Unknown>”?

  1. keul125 says:

    The “System Volume Information” or others forlders can be readed in admin mode by adding reading file/folders rights to the folder (in the security pannel of the property box of the folder, add all user, with read rights)

    Also remembre to use scandisk to correct file system errors, it was my case for an 1Go of ^^

  2. Oliver says:

    Well, there are other ways to read this as well without changing the security permissions on the file(s).

    If you use NTFS you will rarely have problems with file system inconsistencies. This usually only happens with FAT/FAT32 and there more often on Win9x/Me than on NT/2K and so on.


  3. dazster says:

    I have a 25Gb chunk of “unknown” memory which I would so dearly like to reclaim for myself. I previosuly had SRP enabled (up to a maximum of 12% of my HDD) – when I disable it the unknown area still remains. It’s a bit of a joke having a 50Gb HDD and half of it being wasted by this Windows “feature-ette” (big thanks to Mr Gates et al).

    I have Lenovo recovery and restore so do not really need the Windows XP restore facility. Do I take it then that the only way I can reclaim this space is to re-format my HDD and re-install Windows XP and start all over again with SRP disabled?

  4. Oliver says:

    I am not sure as I don’t have access to your software. But, it might have to do with this Lenovo software. I know from other backup software that they are keeping a “secure zone” which is either a special protected area on the system partition or an extra partition. In your case the first one sounds reasonable. Try to find more information about this program.

    Also run chkdsk /r /f C: (modify C: for each HDD partition on your system), in case there are file system inconsistencies.

    // Oliver

  5. dazster says:

    The lenovo software is managed in a separate 10Gb partition – nothing to do with this 25Gb unknown space.

    I have tried running chkdsk and can see no other hiden partitions. Also, the results from chkdsk agree with what I’m seeing in explorer – i.e. far more disk usage than expected. I have also defragmented the hard drive to no avail. I am seriously considering running PartitionMagic on it and attempting to reclaim this space and use it as a Linux partition. I wouldn’t mind so much if it was a few hundred meg – but 25Gb?!

  6. Oliver says:

    Fragmentation could cause such a waste only with huge cluster sizes and very many small files, so this can be ruled out.

    So you ran chkdsk with the command line I suggested above to force it to check for and repair errors?!

    Well, that’s basically the last idea. I have no clue then what it could be and it is hard to diagnose from remote anyway.



  7. dazster says:

    After some investigation, it turns out that this WAS to do with the Lenovo backup facility. I was able to play a little with the facility and thus reduce this space by deleting all previous backups and creating a new one which does not include the larger, non-essential files.

    Thanks for your feedback.

  8. Oliver says:

    Alright. Great to hear that.

    // Oliver

  9. Dan says:

    My situation regarding “unknown”:

    The pagefile always is reported correctly, btw at around 300MB to 800MB.

    The doesn’t always appear after running WDS.

    is always exactly 3GB when it does show up.

    The 3GB does not figure into actually counting the WDS reported free space with “all the rest” of the C: drive’s files (which is 40GB)

    I have a 4GB HD and with this 3GB of “unkown” – by merely adding – I have 43GB, and that would be impossible.

    Today I ran chkdsk from a DOS Window (OS is Windows2000Pro SP4), re-ran WDS – all on a hunch – and the 3GB extra unkown space vanishes from the WDS report.

    I only run WDS as an Administrator, Hobernate is OFF, and so also just 1 partition.

  10. Oliver says:

    A newer article which summarizes the whole procedure in three simple steps can be found HERE.

    // Oliver

  11. Brady says:

    Ah that makes sense. I recently bought a 1.5TB HDD and theres 217GB of . Now that I know what it is, time to fix that problem. if you do the math, its 16% of my total space…that’s a lot considering the drive.

  12. belltelphoneman says:

    I have 51 GB of unknown in my Vista machine. My HDD is 372GB so it is no hardship so far. If it is restore files and I am going to check I can probably change the size of the alloted memory for this. Thanks for the info on this page.

  13. jacob says:

    the unknown showing up on mine is my own data that i can no longer access πŸ™

  14. Curt says:

    I have 200GB of unknown. wtf.

  15. John W Hirsch says:

    Thanks for the assistance…I ran WDS as an ADMINISTRATOR…that made all the difference in the world…now there is very little ‘unknown’. The file causing my freespace to disappear was an SQL log file. Use SSMS to ‘shrink’ the log file. I regained over 40 GIG in freespace.

    this is Windows 7, MS SQL 2008

  16. john says:

    i have solved it, i had 144gb unknown space on my hdd, follow this see if it will work for you. go to control panel. admin tools. comp management. storage. disk management. click on the drive with unknown space, ie “c”. right click on it. select properties. do a disk clean up. when its finished then click more options, then click on system restore and shadow copies. click clean up.then click on deleat but all recent restore point. go back to disk clean up and click ok. then click deleat files. when finished exit all programs, like me i hope you got your unknown space back. if this helps anybody, please send me a message. glad to help. john…

  17. Oliver says:

    Glad you found it. Specifically the volume shadow copies remain a mystery. I don’t know how to enumerate them (in order to display them in WDS).

  18. Frank Jensen says:

    Hi there.

    I have tried all threads found, cleaning up, and it dont work for me. I still have more than 60% (95 Gb) unknown area.

    – I tried disk cleaning
    – Deleting restore points (there arent any, has never been enabled)
    – Running scandisk (no errors)
    – Running MalWareBytes (no malware)
    – Running full scan McAfee (has never reported anything)
    – Scandisk seem normal

    What is really strange is, that the first times windows reported low diskspace, I deleted some files – but fairly quick within a couple of days, that free space of a couple of gigs, was also absorbed by the unknown area – this has been the same a couple of times.

    I have no data on this PC, that I do not have a backup of, but it would annoy me, not to know, what is going on. And then there is the matter of WIN7 and the licence, that can no be re-activated (as far as I know). Its WIN7 PRO 32 bit. NTFS.

    Any ideas ?

  19. Sunny Ahuja says:

    I am having 172.6 GB of “Unknown” Crap Guys Help me out Its not mb but GB by god..

  20. Sunny Ahuja says:

    Vola Got it done it was Windows Backup 142 Gb was used by Windows Backup And 33 .31 GB by System Image .. Well Guys who havent found the solution do check Windows Backup Instead of running other disk checkers

    Save Time


  21. Mary says:

    My computer ran out of space and one-day suddenly all of my files were gone. I have 99% of them backed-up, with the exception of my QuickBooks files, which I really need. I ran WinDirStat and can see that all of the files are there but can’t retrieve them. Does anyone know how to retrieve them? Is there something that works with WinDirStat to allow you to access the files? Any other suggestions?

  22. Austin says:

    I must say, out of all the strange files, folders, and exploits I have discovered with WDS, (black box “files” folders, not-so-empty recycle bin, etc.) the unknown phenomena would have to take the cake (unless you count shadow copies). My initial thoughts were a virus of some sort, (was taking up 80gb) and that moved to a large sum of corrupted data, which was somehow increasing.in size. After reading this blog, I discovered that it was 2 system restore files. Thank you so much for showing me how to clean this up. Kudos everyone. Especially you WinDirStat.

  23. Tony says:

    I found 19GB of space in WinDirStat on my external hard drive. It was also showing as ‘unmovable files’ in Auslogics disk Defrag. After some investigation I found 2 huge system restore point files hidden in the System Volume Information folder where I also found a file called MountPointManager. None of these can be deleted even with Admin privilege, but can be removed in the System section of the control panel – viz – Control Panel>System>System Protection>Available Drives>(Select Drive) > Configure>Delete all restore points. Done.
    You need to turn off system protection to avoid them from coming back.

    Hope this helps you save time !

  24. Gerald says:

    On my 286GB HD, I apparently have 191GB of taking up very valuable space. I’d be very grateful if i could get any amount of help in getting back my disk space, I’m down to only 4 GB!

  25. Robert says:

    I had 131 GB of . Wow! I have Win7.

    I did the following:
    Right-click C drive –> Disk Clean Up –> Clean Up System Files (this gave me an “Options” tab) –> Options –> Clean Up (under System Restore and Shadow Copies)

    Poof, I got 131 GB back. Amazing that Win7 was sucking all this up with no notice to the user.

    Thanks all for the help above. It lead me to this.

  26. Oliver says:

    Robert, I think that Win7 will remove the gradually, except for a minimum amount, as soon as your disk keeps filling up and the SRPs are in the way.

  27. jim mcmurchie says:

    Anyone have ideas where the problem is occurring on a server machine rather than a desktop? Running server 2008, so no restore points, not configured to run shadow copies. Love some help there is currently 170GB unknown.

  28. Suppadean says:

    I ha the same problem. I was logged in under my domain user. I had 60GB unknown. I logged in to my local administrator account and then I only had 1.7GB unknown. The 60GB was my profile for my local account user. The 1.7GB unknown is from my domain account. It seems WDS has a permissions problem.

  29. CaptainMiniBar says:

    Wow!!!!! Sony Vaio twin 128G SSD Win7 Pro. Got popup stating drive was nearing full. Impossible – Win takes 23G, my personal stuff 13G and assorted programs 15G. Checked WinExp and showed SSD almost full. Found both WDS and Spacesnifter. Employed both. No help (am not a computer wizard but smart enough to know what I am dumb at).

    Called Sony – they said it was a software problem. Called MS and paid $99 for investigation and repair. THEY couldn’t fix it and returned my $99.

    PC World Magazine steered me to your site and blog. Read may where System Restore was causing problems. Checked SR and found three restore points and 5G allocated to SR. One poster turned SR on and off and recaptured his unknown space. I did the same and so did I!!!!!!!

    Thank you all!!


  30. Klandaika says:

    I had the same problem (obviously). I’m running Win7.
    I tried all the suggestion in this and other posts, and nothing helped. I never had any backup software on my system either. So I decided to take a different approach.
    Since many people mentioned that lack of permissions was causing this to happen, what I did is go through EACH and EVERY folder on my system taking ownership of every folder that I had no permissions to access.
    Lo and Behold I found it. The entire chunk of unknown space was used by the folder C:\windows\CSC. Quick Google search of the purpose of the folder revealed that windows caches files whenever a user works in remote environment such as accessing shared files on other computers/servers (even if offline files are disabled). And since i do that quite a lot on the daily basis, the folder has accumulated 66 Gigs of data.
    I’m glad I found it, cause’ like many others i was worried that it might be some sort of virus or something…
    Hope this helps

  31. FJ says:

    I cound not get anything to work. The size of my System Volume Information folder was huge, no matter what trick I did from within win7. Deleting restorepoins only removed a fraction. Eventually I booted up in safe mode with commandprompt, and was able to compeltely delete the System Volume Information folder. And then my space was back. Windows automatically created a new folder, that now contains 0 bytes. This is half a year back, and the folder still is 0 bytes, cause restorepoints is disabled.

  32. CJ says:

    Hey people. I’ve got 238.6GB reported for . Any suggestions?
    I’ve cleared my system restore points, ran chkdsk couple of times, used disk clean up and clean up system files.

    I’ve scanned my computer using mcafee and malwarebytes.

    I’ve accidentally pulled my plug while watching a video on my external hard drive, not sure if it helps…

  33. CJ says:

    reported for Unknown*

  34. I have 200MB “Unknown” on a 4GB FAT32 Truecrypt drive. There are no restore points (never enabled) or Recycle bin, or backups or anything like that. I ran WinDirStart in Admin mode.

    Might the Unknown actually be slack space due to many small files and large cluster size?

    In that case, it would be nice to see an update to WinDirStat. Though I have very little hope for that, as I haven’t seen an update since… 2009? Looks like the author gave up.

  35. Max Malm says:

    340 GB unknown here πŸ™‚

  36. whbecker says:

    You might try spacesniffer. Here’s a link:
    Run as administrator and you can see into those “black holes” of WinDirStat. Once you know what’s in that space you can sort out how to free it up.

  37. Nagan says:

    On running WDS in my Win 7 x 64 system I get some irritating “BLACK BOXES”as icons in place of blue icons for Unknown , Free spaces , Files. Pray why the original icon is not present.

  38. Rolando Tabora says:

    I have a 2TB drive (Drive D:) that when formatted comes out to 1.81TB or so. I download a lot of stuff in this drive and was quite surprised that in a relatively short time I was given a ‘Low Disk Space’ warning. I would delete some files and the system would report, say, that I have 100GB of free space – then just a couple of hours later, I’m low in disk space again when I know I downloaded only 10GB. So where did my 90GB of freed space go?

    I used ‘windirstat’ and turned on the ‘unknown’ option on. It reported that my actual used space was 1.1TB! The ‘unknown’ part was taking up 750GB. Tried Disk Management, CheckDisk, Restore, etc. Nothing worked.

    There was only one thing I haven’t checked – Backup and Restore. Turned out that I have turned on scheduled backup on that particular drive (was that a pun?) and have forgotten all about it. Sure enough, after checking the backup configuration, I had a 750GB of backup eating up my drive! That’s about 40% of my drive.

    So, I simply turned off the backup and I recovered the 750GB as free space. I’m currently reviewing my backup options.

  39. Pies says:

    If you’re having a lot of space under Win7, try running WinDirStat as Administrator (right click -> Run as Administrator).

    In my case there were 20GB in the C:/Windows/Temp directory, which is quite a chunk of space on a 75GB SSD.

  40. Yoda64 says:

    I had the same problem on a Windows 2008 R2 Server. The server is a DC with no shares and no system restore. I found that one of my fellow “IT Professionals” had set the security log to archive. What happened is that over time, the archives just kept growing. Its strange how Winderstat showed it as an unknown file and the actual files were plain as day once you knew where to find them. The key was to use SpaceSniffer to get more information on what the files were.

  41. Wolfgang Preuß says:

    I encountered also the problem of a huge ‘Unknown’ part (more than 1/3 of my disk) shown by WDS, and this even if I was logged in with administrator privileges (under Windows 7 SP1 prof. 32 bit). After having read one of the remarks in this blog I started WDS explicitely with administrator rights: and now the ‘Unknown’ part occupied only 1% of my disk!
    Thanks for the hint!

  42. Pingback: HUGE freaking .Cab file on my SSD - Page 2

  43. Charlie b says:

    Hello everyone,
    I found another major contributer to UNKNOWN. I would never have suspected “disk compression” to do it. Use it only sparingly. one example was I compressed “my recent documents” in the user folder, it contains only 803 shortcuts. upon compressing this i gained 0bytes in disk space and lost 55mb to unknown. i reclaimed 700mb disk space by uncompressing more.


  44. Charlie b says:

    I was using XP, by the way.

    The URLs commenters put behind their names can contain some interesting things πŸ˜‰

  45. orin says:

    I was using Data Shredder 1.0 (actually is ALWIL Software’s Shredder found on BART CD, Hirens….). Every time i ran it to CLEAN FREE SPACE(on partition D) with option to clean cluster tips, it gave an error at some time. After that, some of the amount of free space was now “GONE”.
    windirstat shows “unknown” around 8 GB(last run of shredder). When it runs, the shredder creates at some point a bunch of files on d:\, each sized around some hundred MB(200 MB for ex). After the error it closes down, and those files are deleted automatically, BUT the space not.
    Q: How can i “find”, and delete, or recover that space?

  46. orin says:

    wonder why you haven’t published my first comment! Are you unconfortable with the names: alwil or bart cd ? Or what part of my comment had you disagree!

  47. Oliver says:

    I merely don’t check too often, and I think in your case not even received a notification about your comment. I’m not censoring as long as it’s related to the topic.

  48. Pingback: <unknown> disk space on WS2008R2–vssadmin command | Chris Conklin's Weblog

  49. Sajith says:

    Hi ,

    I have a different issue… My Vhd File Size is only 30 GB where as the partition inside it has about 70GB of Valid Data. Any comments on this ?


  50. Oliver says:

    Well, my guess would be it’s either sparse files or the actual partition within doesn’t contain 70 GiB.

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