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remove unallocated space from a partition image

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linux-howto

http://unix.stackexchange.com – 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? (HowTos)