Why does my C Drive Keep Filling Up? - Explained

Dennis Faas's picture

Infopackets Reader 'Old Techo from Oz' writes:

" Dear Dennis,

I have consistently noticed on 3 of my Windows 7 PCs that the C drive appears to bloat over time with 'low space' warnings constantly ... For example, Windows Explorer reports my C drive full with 78 gigabytes of data and only 6 gigabytes free. I knew this had to be wrong ... The only solution I have found is to make a disk image backup of my hard drive partition (using Acronis True Image, for example), and then restore the disk image later on. After the image is restored, Windows now reports correct free space available of 37 gigabytes ... Clearly something weird happens over a period of time (a few months) that results in the c drive is filling up. I'm tired of these low space warnings! Why is Windows reporting incorrect space available? If it's not that, then why does my C drive keep filling up mysteriously? "

My Response:

This is a frequently asked question -- and you are correct, it is a bit of a mystery as to what causes the c drive full / low space warning error messages to appear. Running a utility like 'crap cleaner' will help to delete some temporary files and free up some space temporarily, but the low space warning will eventually come back because you may not be fully addressing the issue.

As such, I've decided to put together this article which will explain why and how the C drive fills up (seemingly mysteriously), and what you can do to reclaim space. Please read the entire article from top to bottom as it is cumulative.

The Definition of Gigabytes and Terabytes

Before I can get into the meat and potatoes of this article, I need to explain something about hard drive geometry with respect to available free space, and why Windows might "misreport" how much space is available to you.

In the past, I've had users email me and ask why Windows is reporting only 931 GB of usable space on a brand new, freshly formatted 1 TB hard drive. The reason for this is that humans view a 1 TB ("terabyte") hard drive as: 1 TB = 1,000,000,000,000 bytes = 10^12 bytes = 1000 gigabytes, as these are nice round numbers. However, computers see a 1 TB hard drive as a tebibyte (TiB); that's because it uses a binary prefix of 1024^4 (which is the most correct number), which equals approximately 0.9095 tebibytes (TiB), which equals 931 gibibytes (GiB).

As such: a 1 TB hard drive is roughly 931 GB, which makes it seem like 69 gigabytes has mysteriously gone "missing", but technically it's not. So, this needs to be taken into consideration when comparing data stored on the hard drive with respect to total / available free space.

Also, when it comes to viewing available free space on a hard drive, it may be reported differently depending on the program you're using. The reason for this is due to the rounding off of numbers, and the fact that there is no clear definition of "gigabytes" or "terabytes" (especially when it comes to how hard drives are marketed). So please also keep this in mind.

Operating System Overhead

Something else to consider with respect to available free space on a hard drive is the operating system overhead. It consists of a number of "invisible factors" that can eat away at hard drive space, and varies from system to system. Operating system overhead can be classified into a number of headings, including (but not limited to):

  • The Swap File and Hibernation File
  • Temporary and Cached Files
  • Windows Restore Points
  • Volume Shadow Copy Service

All of the above factors combined can eat up a tremendous amount of space on the hard drive. I'll explain why, and how you can reclaim the majority of that space.

The Swap File and Hibernation File

The swap hibernation files are directly related to the amount of RAM installed on the system. For example: if your computer has 16GB of RAM, then the swap file will take up (by default) 16GB on your hard drive, and the hibernation file will use up around the same. Thus, you will "lose" approximately 32 GB of space on the hard drive from these two files alone. Note that both files are stored on the hard drive as secondary storage to supplement RAM; you can disable the swap file if you like, but I don't recommend it.

With respect to your original question: if you make a disk image backup of the C drive, both the swap file and hibernation files will not be backed up (this is done on purpose to save space on the backup). When you restore the image, only the swap file will be recreated the next time you reboot, and the hibernation file will get recreated the next time you hibernate. Thus, it may appear that you will have more available free space after you restore a disk image (initially).

Temporary and Cached Files

Many programs use temporary and cached files that are stored on the hard drive. For example: when you install a program, temporary files are unpacked and written to a temporary directory on the hard drive (usually c:\users\yourname\appdata\local\temp). Over time, this can eat up a lot of space if you don't clean out your temporary directories.

Cached files are also stored temporarily on the hard drive (usually from a web browser), so you don't have to keep downloading the same files over and over from your favorite websites, thus speeding up your browsing experience. Cached files can also eat up a lot of space over time.

You can free up temporary and cached files from within Windows by accessing the Disk Cleanup. To do so:

  1. Click Start and then type in "Disk cleanup" (no quotes).
  2. Wait for the Disk Cleanup icon to appear, and then click it.
  3. A new window will appear. Choose the C drive from the drop down list.
  4. Windows will analyze the drive; the "Disk Cleanup" window will appear.
  5. The first tab labeled "Disk Cleanup" allows you to remove temporary files. The second tab allows you to delete Restore Points and Volume Shadow Copies, including older Windows backups. Both these options can save a significant amount of space (especially Restore Points).

I'll explain more about Volume Shadow Copies and Restore Points below.

Windows Restore Points

As I mentioned above, Windows Restore Points can eat a tremendous amount of disk space over time. Restore Points are effectively mini 'backups' of the operating system (consisting of the Windows Registry, mostly) and are usually created when you install a driver or use Windows Update.

Recently my brother-in-law complained that the free space on his hard drive had diminished at an alarming rate over a short period of time. He later found that his Windows System Protection settings (which are used for Restore Points) was set to accumulate up to 50% of his hard drive. He lowered the limit to 5% and reclaimed roughly 300GB of space. That is a very significant amount of space to reclaim!

I highly recommend you review your System Protection settings and adjust it accordingly. Setting this value to between 5 and 10% should be more than enough. To do so:

  1. Click Start, then type in "create a restore point" (no quotes).
  2. Click on the option "Create a restore point" when it appears in the list.
  3. Under the Protection Settings heading, select the C drive and click Configure.
  4. Under the heading Restore Settings, ensure "Restore system settings and previous versions of files" is selected. This will enable System Protection Restore Points AND Volume Shadow Copy Service (described further down).
  5. Under Disk Space usage, drag the slider to either 5 or 10%, depending on the size of your hard drive.
  6. Optionally: under the heading "Delete all restore points ...", click the Delete button to reclaim space on the hard drive. This will most likely reclaim a significant amount of space.

Personally, I disable my Windows Restore points and instead use Acronis True Image to backup my operating system. Restore points don't always work because they only backup portions of the operating system. In comparison, disk image backups are effectively bullet-proof backups which contain the entire operating system. If you're using Acronis True Image already, you most likely will want to disable the Windows System Protection as this would be redundant and eat up space on the hard drive, which is not necessary.

Volume Shadow Copy Service

The Volume Shadow Copy Service keeps older versions of files on the hard drive; this is a very useful feature if you accidentally overwrite a file. By default this setting is enabled within Windows if you also have Restore Points enabled. You can only disable this setting if you Turn off System Protection completely (the steps to configure this are listed in the previous section).

If you have System Protection enabled (which also enabled Volume Shadow Copy service), you can reclaim space used by Volume Shadow Copy by following the directions I listed previously under the Temporary and Cached files section.

A Dirty File System

Keeping in mind all that I have mentioned above, the only way that a hard drive can possibly misreport available free space is if the file system as dirty. This is usually caused if you lose power and if CHKDSK (check disk) wasn't run on the next reboot of the computer. You can fix this by running check disk manually. To do so:

  1. Click Start and then select Computer or "This PC".
  2. A new window will appear; left click the C drive to select it, then right click over top of the selection and choose Properties.
  3. Go to the Tools Menu and look for the Error Checking heading, and click the Check Now button.
  4. A new window will appear; ensure "Automatically Fix File system errors" is check marked, then click Start.
  5. You will need to reboot your computer to complete the check disk.

I hope this answers your question and sheds more light on why the C drive becomes full, and how to go about freeing up some disk space.

Additional 1-on-1 Help: From Dennis

If all of this is over your head and you need help organizing your data and/or freeing up disk space to get rid of that pesky "Low Space" warning message, I would be more than happy to help you using remote desktop support. Simply contact me using the contact form, and we'll set up a time to meet and discuss your options.

Got a Computer Question or Problem? Ask Dennis!

I need more computer questions. If you have a computer question -- or even a computer problem that needs fixing -- please email me with your question so that I can write more articles like this one. I can't promise I'll respond to all the messages I receive (depending on the volume), but I'll do my best.

About the author: Dennis Faas is the owner and operator of Infopackets.com. With over 30 years of computing experience, Dennis' areas of expertise are a broad range and include PC hardware, Microsoft Windows, Linux, network administration, and virtualization. Dennis holds a Bachelors degree in Computer Science (1999) and has authored 6 books on the topics of MS Windows and PC Security. If you like the advice you received on this page, please up-vote / Like this page and share it with friends. For technical support inquiries, Dennis can be reached via Live chat online this site using the Zopim Chat service (currently located at the bottom left of the screen); optionally, you can contact Dennis through the website contact form.

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f58tammy's picture

Thank you for this response, I am wondering if you could give more detail about System Files in Disc Clean-up. I just installed the Windows 10 build 10130 and in system file I have 3 big ones, Device driver packages 7.5GB (which I believe would be safe to clean), Previous Windows installation(s)18.7GB (which are probably necessary to rollback to the previous build), and Temporary windows installation files 3.47 (these I am not sure if they should be cleaned or must keep). Thank you for your expertise in the articles you write.

ecash's picture

There is allot not mentioned here..
Old RULE...Never use the last 10% of the drive.
Its an old rule, but the last 10% is abit important. If you get to that point..Erase, clean up, MOVE stuff off the hard drive..And yes, its a BIG chunk..1tb drive, 10% is 100gig..
Another think, is that games that you Load onto your system may not Erase the Compressed files..

jonathan.powell_4678's picture

How about downloads for endless security updates? These do not seem to vanish on my PC's, and I'm a bit unsure about deleting stuff of this nature. Often hard to identify. I recently purged about 15GB out of necessity on an 80GB boot drive.

lelandhamilton's picture

Disk drive manufactures quote drive space as powers of 1000 so that a 2 TB drive is 2,000,000,000,000 bytes (2e12, 2*10^12). However most software reports disk space as powers of 1024 and considers 2 TB to be 1024^4, 2,199,023,255,552 bytes. As the article points out the term TiB has been invented to reflect 1024^4, but this is the first time that I have seen it.

But there is more to the story than the items identified in the article.

The disk manufactures space is quoted for an unformatted disk. Some space spread through the drive may be reserved for hardware bad block reallocation, hidden from the OS (in addition to the OS bad block processing if hardware bad block reallocation fails). Formatting the disk reserves some bytes at the beginning of the disk for boot blocks, partition tables; and various other metadata files including the Master File Table and a bitmap of allocated/in use clusters. (Refer to http://www.pcguide.com/ref/hdd/file/ntfs/arch.htm for more detail.)

NTFS reserves an initial amount for the Master File Table and about 12.5% of the disk space immediately following the MFT for metadata, but this can grow as the number of files and directories increases. If the disk gets really full, some space can be allocated in the reserved MFT space for ordinary files, but if your disk is that full it is past time to do some housekeeping.

A cluster is the smallest allocable and addressable amount of disk space. In the early days cluster size was 512 bytes but as disk sizes increased the cluster size increased to accommodate mapping the increased sizes. 4 Kb is the default cluster size for current OS versions. (4K happens to be the largest size that the OS compression software can process. Some older OS may have used larger cluster sizes on large disks.)

Another apparent space discrepancy can occur in NTFS: small files that can fit in the remaining portion of the cluster after a file’s MFT entry will be placed in the MFT entry instead of allocating additional clusters for the file.

Now you can subtract the space for the other things mentioned in the article:

The Swap File and Hibernation File
Temporary and Cached Files (including browser caches)
Windows Restore Points
Volume Shadow Copy Service
Let’s not forget
Trash bin

By the way: I recommend a fixed page/swap file size: 4 GB or just under for 32 bit Vista and older Windows systems (even if you only have 2 GB installed memory). The larger swap file sizes ease some of the burden of finding “virtual” memory to load programs, and I usually leave programs in memory instead of exiting and restarting. If the swap file is too small, loading too many programs to run can cause lots of page file faults (thrashing) as programs compete for memory. (This thrashing from a small swap file has occurred in other operating systems for multiple decades, including Linux/Unix and a number of other systems.) Have not had the (dis)pleasure of using a newer OS, so no size experience there. However I might suggest an initial minimum swap file for 64 bit systems to be at least 2 to 4 times the installed memory. If the system gets sluggish a system monitoring tool should be able to identify the page fault rate and other system history.

rohnski's picture

I just "tripped" over this article, found the link while reading a newer one that referred to it.

I will be passing the link on to lots of people. In the last month or so I've been seeing a common trend of questions from people who can't apply the Fall Creators Update because they don't have enough free space on their 32GB portable devices <sigh>. It's a shame that MS says the minimum spec for Win 10 is 16-20GB. That makes a 32GB drive adequate for the initial install. But it does not take into account installing applications and personal data files and it most definitely does not allow for all of the space wasted by MS Updates to Windows.

I keep hoping that some media ombudsman will take up the cause and start the ball rolling to get the OS (Win/Apple) to report file sizes in "normal" numbers, Decimal.

I hope you will invest a little more time in this article, update it and republish it.

binary vs decimal - update

In the first section where you talk about the difference between HD marketing in "inflated" decimal sizes, while the OS reports in binary. Historically, I see the reason for the OS resorting to binary reporting file sizes. But I am NOT a computer, counting in decimal is ingrained, counting in binary is unnatural. I think it is time for the OS to bit the bullet and start reporting in decimal. The rounding error from binary to decimal on individual files would be trivial.

Back when I bought my first computer I spent an extra $500 to "upgrade" to an 80mb drive. But when I pay for 1TB I am "losing" 69GB which is 1 order of magnitude greater than that original disk I paid extra for! For that money now I can get 1TB of SSD or up to 5TB of spinning disk. The thing is, back then the difference between binary and decimal reporting was under 5%. Today, dealing with TB decimal is inflating the size by 10%. It is only going to keep getting worse. Wikipedia has a good page on the whole Binary vs Decimal counting thing:

Half way down it has a chart showing the percent differences between the various orders of magnitude. That page also lists all of the names and abbreviations for the orders of magnitude.

If you do update the article there are some more subjects I hope you consider adding.

- clean the AppData folder for applications that have been uninstalled.

- remnants of uninstalled programs in the "Program files" and "Program Files(x86)" folders.

- Windows Update uninstall files (diskclean)

- remove the recovery partition

- You mentioned a couple of caches, this article lists 7 that can be cleaned up to free up space

- clean up Office update files https://winaero.com/blog/free-about-several-gbs-of-disk-space-after-installing-office-2016/

- Side by Side SxS files http://www.davescomputertips.com/how-to-regain-hard-drive-space-new-windows-disk-cleanup-features/