windows 7 - What is the difference between "used space", "size" and "size on disk"?

There is only one partition for both system files and personal data, which is C:. The other two partitions are for boot and for recovery.

Now I have two different reports on the size of C: on my Windows 7:

1. by clicking the property of C:, it shows
   59.4 GB for "used space";


2. by selecting all contents (I have setup to view all hidden files and directories) under C: and
   viewing their property, it shows
   46.9 GB for "size" and 47.6 GB
   for "size on disk".

Here are my questions:

3. I was wondering why the two reports
   are not the same, and where the
   difference comes?


4. in the second way, why "size" and
   "size on disk" are different? What
   do they mean actually? What is the
   
   difference between them and "used
   space" in the first way?

Thanks in advance!

Answer

1. There's some files and folders that you don't have access to, even as Administrator. System Volume Information is one such folder.


2. You can't directly see some items that take up space, like the MFT and USN Journal.


3. On Vista & 7, the Windows directory is densely hardlinked. Explorer is unable to handle this and will count files with n hardlinks n times.



4. Explorer doesn't take NTFS Alternate Data Streams into account.


5. Files locked for exclusive use won't be counted either.

**No matter what you do, you're not going to be able to select all the files/folders on the drive and make the number match the drive's properties.

As for "size" vs "size on disk", there's several things that make the two differ. "Slack space", as mentioned by the others are the most common. But files stored with NTFS compression may take up less space on disk, affecting Explorer's display. Sparse files, offline files, and other special attributes can also cause the 2 to differ significantly.

**You can use a linux based os or a live cd to see all the hidden folders.
For Windows 7 one of those folders is called ProgramData and it accounts for the big difference in size reports.