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I have just created an image of an sd card using dd and the image is compressing down less than expected. I suspect this is because the card had not been zeroed out before I started to make the image. Is there a way to clear out "junk" stored on an image without having to start from scratch?

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The easiest way to zero out unused space is to mount the image, fill up the space with zeroes, and remove the zero-filled file.

mount -o loop /path/to/image /mnt
cat /dev/zero >/mnt/zero
rm /mnt/zero
umount /mnt

Depending on what the filesystem is, there may be tools that can work directly on the image. See this question for ext2/ext3/ext4 (answer: zerofree).

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