Windows explorer displaying wrong free space

I have a 1TB* external HDD which comes out at 931GB total space on windows explorer (run as admin).

When I look at free space it lists only 183GB free with 747GB used. However, there is not this much used space. When using WinDirStat (run as admin) this drive only uses 472GB of space.

I have cleared the shadowstorage down to the latest shadowstorage using DiskCleanup which increased the free space to the 183GB free. Now I can't think of anything else that could be causing this 275GB discrepancy.

I had used this external drive on an old laptop, could there be an issue with that?

Thanks for any advice.

WinDirStat may not be able to open some folders even when run as administrator, for example System Volume Information folder which also consume space.

NTFS has also Alternate data streams feature. It's not widely used, and usually software does not take that into account when displaying file size.
 

• To see how much space is allocated for Shadow Copies, run

  vssadmin List ShadowStorage
  

  Its outputs looks like this:

  Used Shadow Copy Storage space: 27.419 GB (9%)
  Allocated Shadow Copy Storage space: 27.759 GB (9%)
  Maximum Shadow Copy Storage space: 28.237 GB (10%)
  

• To see how much space is used by system, run chkdsk in read-only mode (replace C: with your drive letter):

  chkdsk C:
  

  And look at numbers in lines with in use by the system and occupied by the log file in the report:

  296082431 KB total disk space.
  119089088 KB in 656918 files.
     407696 KB in 80692 indexes.
          0 KB in bad sectors.
     894379 KB in use by the system.
      65536 KB occupied by the log file.
  175691268 KB available on disk.
  

I have finally solved my problem and would like to thank everyone for their help. As I said in my opening statement this external was used on an old PC and for some reason this lead to permissions issues. After going through all the subfolders to change the permissions for the files the drive's used space is now correct on WinDirStat and when selecting all the files and clicking properties.

@Psycogeek - you were correct in saying that MyDefrag shows the files, thanks for that, unfortunately due to the files being in a folder that I had used frequently and had permissions to I wasn't able to determine that they would be missing. Plus 675GB of files is hard to go through :P

@Alexey Ivanov - The files were viewable in windows explorer like you said but WinDirStat and windows explorer apparently do not include files with permissions from an unknown source - thanks for the help.

If anyone else has this problems it could be down to this issue. File explorers and visualisers seem to have trouble examining files that have different permissions - You have to remove the unknown user's permissions and change the permissions to your current system - in my case this was arthur-PC/arthur rather than a long string of characters. As there are already guides for this I will post a link.

http://answers.microsoft.com/en-us/windows/forum/windows_vista-files/how-do-i-change-folder-and-file-permissions/465f2b42-63dd-4486-8dd1-c870290efeed

First answer is correct. - remember to click subgroups and folders, because I did not do this when orginally changing permissions I could view the contents of the folder in windows explorer but not subfolders and you have to click each and every one to gain access to it.

Thank-you to everyone for your help, I hope this answer can help people that might have the same issue.

Arthur