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I have a new MY Passport hard drive 1 TB, Model: WDBBEP0010BBk-EESN. I checked the properties of the hard drive when I first plugged it in, the used space was 320 MB, though the total size of the data was 191 MB which is the software that comes with it (of course I show all hidden files and hidden operating system files). I done a quick format to the drive, but the space is still used (129 MB). I copied some (2.69 GB) of data on the drive and then deleted them, the used space even increased to be (168 MB). I've done quick format on the drive again, the used space returned to be (129 MB), I then copied the same data (size: 2.69 GB, used space: 2.93 GB) then removed it, the used space increased again to be (168 MB). I then copied data to the drive with size 19.6 GB, the used space became 20.2 GB.

I deleted all of that data to make the drive empty again, the used space increased further to be about 300 MB. It seems that the more I add data the more I lose space, Weird? Can any one tell why this could be happening?

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Out of curiousity, how many files were contained in that 2.69 GB of files you copied? –  Breakthrough Dec 1 '12 at 16:15
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You have to remember, there's more that needs to be stored on your hard disk than just the 0's and 1's contained within your files. You need to store the filesystem layout, which has a fairly significant overhead depending on the number of files, and how fragmented the files are (all this depends on the particular file system you're using, however).

By default, the NTFS file system reserves some of the partition for the Master File Table (MFT). This is part of NTFS, since it's a journaling file system. In addition to the MFT, there's also a USN journal to track/record changes to files or file attributes.

This is obviously a gross simplification (there's much more going on in the background), but the main point is that you require more space to store a file than how much just the file consumes. This is why some operating systems (e.g. Windows) provide two disk-use metrics: "size" (implying the literal zeroes and ones making up the file), and "size on disk" (adding the filesystem overhead to the previous figure).

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Ok,but why when I remove all the files on the disk the used speed increase? –  Ramy Dec 1 '12 at 16:19
    
Once MFT space is used, it is never returned to free space. –  David Marshall Dec 1 '12 at 16:20
    
Ok,but why when I remove all the files on the disk after adding different files the used speed increases, more than after the format's used space? And isn't the MFT and the rest of the NTFS files supposed to exist somewhere on the space that's not accessible by the user (69GB), (1TB - 931GB = 69 GB)? –  Ramy Dec 1 '12 at 16:30
    
What do you mean 'used speed'? –  David Marshall Dec 1 '12 at 16:31
    
I think what @DavidMarshall was trying to get across is that once the MFT is resized, it will never be shrunk again. This is to prevent MFT fragmentation and maintain it's integrity. Once the MFT is full, it's size is expanded as per the file system implementation. And the MFT is technically accessible by the user (most parts of the filesystem are), although it's very difficult to access without knowing how (the file is called $MFT; see the part on Metafiles in the Wikipedia NTFS article for more details). –  Breakthrough Dec 1 '12 at 16:32
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