On Wed, 6 Mar 2019 09:12:05 +0000
Andy Burns  wrote:

> R.Wieser wrote:
>
> > I'm intending to do a raw backup (alike DD do if I'm not mistaken)
> > of the SD card on a Windows machine and than zipping it.  For that
> > reason I would like to clear all unused sectors before making the
> > copy, as than the zipping will result in the smallest possible file.
> >
> > Q: Does Stretch have a program available which can do that, and if
> > so which is it ?
>
> 

I'd have done
cat /dev/zero > dummy.file && rm dummy.file
easy to remember and type but apparently a lot slower than zerofree.

The secure delete package has an option to wipe empty space too,
sfill -fz
would seem to do the job, but I'm not sure how it does it.

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