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I have 1 drive in the machine, with 190 GB used. Why is the system image 300 GB (as reported by Windows backup)?

Specs: Win 7 x64 Pro, Windows Backup set to retain only latest image, backing up to external WD MyBook.

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  • Where does it report that? Could you provide a screenshot?
    – Oliver Salzburg
    Jul 15, 2012 at 11:52
  • 3
    Don't understand the down votes, seems to me anyone running backups in Win 7 would want to know this...
    – Henry Lee
    Jul 15, 2012 at 16:56
  • Your question was unclear to me before you posted the screenshot. I reverted my downvote as I now understand your issue clearly.
    – Oliver Salzburg
    Jul 15, 2012 at 17:27
  • I agree thisd shouldn't be closed, is it to late to vote reopen, I know I can't?
    – Paul C
    Apr 12, 2013 at 9:40
  • This is a in important question. Why is it closed? Please open for answers! I want to know this too!
    – Frank Breitling
    Feb 23, 2018 at 9:24

2 Answers 2

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To understand this calculation better, let's look at my local setup.

  • My main system drive (C:) is a 238 GB partition.
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  • My Windows Backup is set up so Windows manages the used space on the backup location automatically.
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Let us note 2 things about what we're observing here.

  1. Neither the 137 GB used nor the 238 GB total seem to come up in the estimates of the Windows Backup disk space management, if you let Windows handle the backup.
  2. I could save 118 GB when switching to the same backup style you use.

So, how much is your current backup actually using?

Good question! Let's look at what Windows Backup tells us first.

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So, it says it uses 462 GB for backups, even though my whole system drive is only 238 GB in size. But this is obviously no clue to your solution, as I keep a backup history and you don't. So, let's take care of that.

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And now my system image only uses 90 GB

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This might now be more confusing than anything else. Why 90 GB? My system drive uses 137 GB!

Let's look at what Microsoft says is even part of the system image:

By default, a system image includes the drives required for Windows to run. It also includes Windows and your system settings, programs, and files.

So, what exactly do they mean by that? I have no clue. But I would assume it includes everything your system needs to be operational (like the Windows folder, the boot manager, your installed programs (Program Files/Program Files (x86)), your user profiles.

OK, this explains why my system image uses less space than my disk usage, but why does yours show more disk usage?

There are a few possible causes I can think of:

  1. You're actually letting Windows keep a history of backups and just didn't realize it.
  2. The data that is being pulled into the system image does not reside on your C: drive.
  3. Something I could not possibly know without sitting at your desk :(

One thing is certain though, the whole time, the full disk image of my main system disk had 90 GB:

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The additional size that was noted in the Windows Backup UI was caused by my own backup history. Which leads me to believe that this must somehow be the same for you.

Old Answer

The system image will be a representation of your whole hard drive. It will include all parts, no matter if they are free or used.

If you don't want to waste all that space, you can backup individual folders or files. You can even select all folders and files, then it will only use the 130 GB.

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  • I added a screenshot of the C drive. The image can't include free space or else it would be 931 GB, not 300 GB.
    – Henry Lee
    Jul 15, 2012 at 17:39
  • @HenryLee: Oh, I see now. Sadly my own backup is running right now, so I can't really play around with it. But, I took the time to dig up what Microsoft says What is a system image?. This might shine some light on your question. Needless to say, my answer was off. I hope to improve it once my own backup finishes.
    – Oliver Salzburg
    Jul 15, 2012 at 17:54
  • If I had to make an educated guess, I'd say it has something to do with default sector size, fragmentation, etc. I think default sector size is 4 KB, so if I have a file that is 1 KB it will still take 4 KB of physical space. So maybe over time my 190 GB of data has come to occupy 300 GB of physical space.
    – Henry Lee
    Jul 15, 2012 at 18:05
  • No, because then your free space would read 631 GB instead of 739 GB. NTFS reports the actual free space -- excluding wasted space in each cluster. Fragmentation also has no effect on available free space. (Note: the OS cluster size need not be the same as the hard drive sector size, though it's more efficient if they are.)
    – taoyue
    Jul 15, 2012 at 18:22
  • @HenryLee: I slightly extended my answer. It's the best I could do, sorry :P Please compare my dialog with yours. You'll see that Windows tells you that at least one previous image is stored at the backup location (possibly from an older Windows installation).
    – Oliver Salzburg
    Jul 15, 2012 at 19:20
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Since Windows Complete Backup produces a VHD, the easiest way to resolve this problem is to mount the VHD and poke around. For example, launch a tree-viewer like Windirstat as admin, and figure out where the space is going. You're running Windows 7, so VHD-mounting is built-into Disk Management.

Either NTFS is wrong, or the backup program is wrong. Probably the latter.

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