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Date: Fri, 19 Nov 2010 17:42:57 -0700 (MST)
From: Warren Block <wblock@wonkity.com>
To: Matthias Apitz <guru@unixarea.de>
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Cc: freebsd-usb@freebsd.org
Subject: Re: copying /dev/da0 with dd(1) to file: output differs
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On Fri, 19 Nov 2010, Matthias Apitz wrote:
> El d?a Friday, November 19, 2010 a las 12:39:13PM -0700, Warren Block escribi?:
>
>>> Should I overwrite the full USB key from /dev/zero?
>>
>> Possibly there would still be differences.  Filesystem metadata like
>> date last mounted, for example.  If you want a block-by-block duplicate,
>> the brute-force method is to just dd the whole drive.  Use bs=64k or
>> bs=1m to help reduce overhead.
>
> Warren, perhaps you missed my point. I have a prepared boot-able key and
> I want to give away a copy of it as a file on DVD. So I dd(1)'ed the key
> to disk and did this twice to ensure that the copy was fine, but the two
> files differ. How can I make sure that the file on disk (or DVD) is a
> exact copy of the key?

Okay, that makes more sense.  Did you mount the key in between the two 
times it was dd-ed?