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 or 3 GB difference between the size and the used space.

Why's the difference so huge (5 GB) ??

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?

Yes


The HDD cannot "mess around" to cause what you are describing - the HDD does not decide what the OS reports as used filesystem space. That is calculated by the OS. Therefore your questions are actually Windows questions in your case, and are not a disk (HDD) problem at all.



That difference is the "slack space" in the NTFS clusters. (You can do some research on that - you will need to learn what a filesystem "cluster" is.)


I don't know exactly whose properties you are checking, so I can't comment on that.

Thanks a lot for your reply..

Here are the details, I guess they will help clear the point a little bit : http://postimage.org/image/jgq9wswsv/

I have little information about allocation unit size, files have a minimum value say of 4096 (4Kb). The data on the hard drive will consist of units of 4kb files even if some files are below 4kb they must occupy the 4kb at least, which wastes some space (size vs size on disk).
I hope you can confirm me right..

What confuses me is the used space in the properties. When I right click on the HDD's one partition and click properties the used space comes larger than the "size", and the "size on disk" by 5GB, and 3GB respectively. there's a 5 GB or 3 GB difference (don't know which to compare to in that case), which is huge.

Thanks again..

Thanks for the link to the image, which clearly shows what you're describing.

Yes, you're basically correct about clusters (allocation units), although there is much more to the story, if you want to look into the details (e.g. NTFS clusters don't have to be 4kB in size; the "slack space" doesn't only exist for files <4kB as you described; some other filesystem types can even split a filesystem cluster to avoid wasting so much space; etc. etc.).

[Edited to add: In case that link to your image becomes stale, I've included the image itself into this post, so that other readers can still see it in the future:]


Looking at that image you kindly supplied - I've already explained the difference between 726GB and 728GB numbers (which were for files & folders (directories) which you had selected, and then displayed their properties). The reported size of used space on the whole partition (731GB) is higher than the 728GB figure because it includes the disk space occupied by any other files which were not selected (e.g. any hidden files, although there may not have been any of these) and also the filesystem metadata overhead (e.g. space occupied on the disk by $MFT etc. on NTFS).

As you can see, these figures do not show a disk problem - don't worry (but always make sure that you have backups of any important data, because disks can fail, or filesystems can become corrupted, without prior warning...)

Thanks a lot Vulcan... This was very helpful..

So 3 GB difference is normal for NTFS file system files? You were right. I had no hidden files, or OS hidden files, (I show them all). Even when I use Linux it shows same hidden files, and gives similar size reports. So isn't that a little big (3GB), I'm thinking of a few MBs?

Regardless even of the fact that when I delete some files not all of there space gets removed with them (the overhead will keep adding up)..
I mean, when I empty the HDD there will still be used space, depending on the data that used to be there, that kind of space only clears if I formatted (If that's normal too, is it??). I'm not talking about removing all of my files, but some of that space that stays there after you remove files, will be added to the (3GB) if I deleted only some files now, and plenty of useless used space will be occupied, don't you think?

This HD is new and I got it to back up an old one that I don't trust

Thank you. Hope you would forgive my humble knowledge of the matter..

I would also wish to add that I checked disk management and I found the overhead 0% if that's the same overhead you mentioned earlier..

No, that 0% figure reported there by Windows isn't the filesystem overhead - it is impossible to have a filesystem with 0% overhead. Think about it - how can a filesystem store information about each file, which it is required to do (e.g. name, size, access/modification times, ACLs, occupied clusters etc. etc.) without using any space to store that metadata?!

I didn't understand most of your earlier post where you talk about deleting files, but I have no more time to do deeper investigation for you at the moment.