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I have a 1 TB hard drive that consists of one NTFS partition which I use to back up my data (no operating system). The size of all the data in it is : 726 GB, size on disk: 728 GB, and the used space when I check the properties is: 731 GB. There's a 5 GB difference between the size and the used space.

Why is that huge difference there?

What's the difference between these sizes? (size, size on disk, and used space)

Is there a way to calculate the difference, and be sure the HDD is not messing around?

Is that normal?

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It's normal. Quick answer:

Size of all the data: sum of the actual bytes of your files

Size on disk: In addition to the above, space waste by clusters/allocation units used by your files. Disks are always divided in small chunks, often 1k to 4k. So a small 0.1K file takes up a full chunk no matter what. So there's always a small loss at the trailing of every file, hence the added size (especially with lots of small files).

Used space: In addition to the above, space taken by the file system. Contains the partition table (dictionary of file locations), journal/log (to prevent corruption in case something crashes during a file operation, a notable feature of NTFS over FAT32), folders, user access permissions, creation/modification/access dates, etc, which must be recorded somewhere too.

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So 5 GB?? Isn't that quite much for tables and file system stuff?? – Ramy Dec 5 '12 at 4:41
User access permissions? Could you explain that please? Are they somehow stored in the filsystem itself? – terdon Dec 5 '12 at 4:41
@terdon Yes they are. See Explains for instance why you are not considered the owner of the files if you plug a HDD in a new system or resinstall Windows (since the associated user doesn't exists anymore) – mtone Dec 5 '12 at 4:48
@Ramy Well it's 2GB for cluster waste, and 3GB for the filesystem, not 5GB. I've added to my answer that it also contains creation/modified/access dates for every file, and probably other metadata. I agree that 3GB is still a lot, but I don't know the specifics. – mtone Dec 5 '12 at 4:54
what specifics you need to know ?? – Ramy Dec 5 '12 at 4:57

This is normal. When the disk is formatted, the space is divided into blocks called sectors. When you create a file, it allocates an entire sector, not the exact amount of the file's size. Dependent on the size of the file versus the sector, you may end up using lots of extra space. Let's say you have a 512K sector size (the older drives use this) and you have a file size of 256k. When you create the file, it must use the entire sector so your file size is 256k, the space used is 512k. The difference is 256k or another way of looking at it is that you have a utilization of 50%. So you can see, it depends on your sector size, your file size, the number of files... You will almost never get an agreement of file size versus disk space used.

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So the actual comparison should be between the size on disk (not the size) and the used space, that represents what's in fact the occupied space?? – Ramy Dec 5 '12 at 4:45

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