the zerofree command finds the unallocated, non-zeroed blocks in an ext2 or ext3 file-system and fills them with zeroes

A NTFS Windows machine, with a mechanical drive, was upgraded from 7 to 10. The drive is old and I suspect that much of the free space actually has data and not filled with zeros.

Is it possible (how?) to zero out the free space, so that when an image is created, the size is minimal?

Assume either a bootable USB configured with Ubuntu or SysRescueCD is available to process the HDD by mounting the NTFS partion with NTFS-3G if necessary

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    you could write a very large file of zero bytes
    – jsotola
    Commented Jun 7, 2020 at 5:29
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    Why do you want to do this? Commented Jun 7, 2020 at 14:25
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    @NonnyMoose Very useful to 1) really disappear deleted file content 2) archive the ntfs partition in compressed form 3) also many VMs are using compressed virtual hard disk images
    – peterh
    Commented Jun 7, 2020 at 22:42
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    Btw, I believe the best option is a dd if=/dev/zero of=/mount/path/bigtmp. This is also an fs-independent option. Although it might be not perfect, for example deleted directory entries might remain allocated, depending on how ntfs handles them. Doing a defrag and then this zero-file is probably close to be perfect.
    – peterh
    Commented Jun 7, 2020 at 23:06
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    Does this answer your question? How to fill a device with zeros, without overwriting the bytes that are already zeros? Commented Jun 8, 2020 at 16:45

7 Answers 7


Depends on the tool you are using to create the image. Usually you don't need to zero it out.

For example ntfsclone (part of ntfs-3g) states this in the man page:

ntfsclone will efficiently clone (copy, save, backup, restore) or rescue an NTFS filesystem to a sparse file, image, device (partition) or standard output. It works at disk sector level and copies only the used data. Unused disk space becomes zero (cloning to sparse file), encoded with control codes (saving in special image format), left unchanged (cloning to a disk/partition) or filled with zeros (cloning to standard output).

So, the free space will be ignored and, if you are cloning to a file, converted to "holes" in the sparse result.

Other cloning software, such as Clonezilla will use ntfsclone by default to create the partition image.

  • That's not 100% accurate. Clonezilla will use ntfsclone or dd, depending on what you choose. Commented Jun 7, 2020 at 18:12
  • I meant by default. Corrected. Commented Jun 7, 2020 at 21:12
  • Strictly speaking Clonezilla will use partclone.ntfs by default, but it will also make an efficient copy like ntfsclone would.
    – gronostaj
    Commented Jun 8, 2020 at 14:05

You could use dd for Windows.

dd if=/dev/zero of=EMPTY bs=128k

Should do the trick.


I BELIEVE this command does what you described zerofree does (namely zeroes out unused blocks/clusters that are NOT already 0) but I have not checked the code / contacted the dev to be sure.

Here is a forum post about suggested changes which I presume were implemented if you want to follow up with the dev : https://forum.tuxera.com/viewtopic.php?f=2&t=30812&view=next

The tool is called ntfswipe and is a part of ntfs-3g, however it needs to be properly configured to get your (and mine) desired result.

I think the following command is correct; i.e. will skip unallocated clusters that are already 0 and otherwise only overwrite unallocated clusters with 0 (by default ntfswipe uses random/list bytes, for "wiping" purposes).

sudo ntfswipe -U -b 0 -v /dev/sdx#

Where sdx# is replaced with the ntfs partition you want to process.


You wrote "Assume USB/CD is available" but can you log into the upgraded Windows system, too?

For my Windows VM, from time to time I use Mark Russinovic's sdelete from the sysinternals suite.

In an admin-CMD enter the following

sdelete -z C:

According to documentation, this will

-z Zero free space (good for virtual disk optimization).

Alternatively, use

-c Clean free space.


Running this after a cleanmgr run in the WindowsVM, the VM-image can be compacted by several GBs.


[EDIT] Since secure erasing is not part of the problem here, this question might actually be a duplicate, the answer of which suggests ddrescue, another great tool, handling potentially failing disks.

Paul gave a good solution on Windows (SDelete from sysinternals/Microsoft).

However, since this question has been posted in "Unix & Linux", I suppose a tool working in those environments is being seeked for.

I would then suggest sfill from the secure-delete Debian package, using:

  • -l to reduce passes to 2 (one 0xff + the last one)
  • -z to replace the last random pass with a zeroing one

Assuming you have enough space to temporarily create a full-size image, you could then mount the resulting image using losetup and/or mount -o loop, then run the fstrim tool against the mountpoint.

On modern kernels, when the filesystem is backed by a loop-device, fstrim will make the image file sparse – the empty areas will immediately stop occupying space and will return zeros when read.

This works for NTFS given a recent ntfs-3g version. It also works for any other filesystem that the kernel has discard support for.

  • 1
    fstrim does not wipe out the data. It simply marks it as not there. It's already marked as 'not there' but OP wants it overwritten with zeros.
    – abligh
    Commented Jun 8, 2020 at 7:13
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    It's currently marked as 'not there' at filesystem level – fstrim reads that information and marks it as 'not there' at the underlying block device level. For SSDs, that translates into a "TRIM" command – but for image files, that translates into a hole within the image. (Linux supports punching holes in an existing file, turning a non-sparse file into sparse.) The result is that the image file will very quickly have those unused areas replaced with zeros. Commented Jun 8, 2020 at 8:06

I am using that script:

#!/usr/bin/env bash
df -HB MiB .
pv /dev/zero >> "$f"
pv /dev/zero >> "$f"
df -HB 1 .
echo -n cleaning...
rm "$f"
echo ok
df -HB MiB .

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