dd errors with certain DVDs

[Charles Curley]

On Sun, Mar 25, 2007 at 11:48:57AM +0100, Paul Smith wrote:
> On 3/25/07, Charles Curley <charlescurley at charlescurley.com> wrote:
> >> I am getting the errors below when I execute the command

> >To answer your other email, you can validate a freshly minted DVD
> >similarly.
> 
> Thanks, Charles. Yes, it was the error regarding dd that was causing
> the md5sums mismatch (reported in the other post of mine). However,
> the method that you suggest works fine, and my problem is solved. The
> command dd is not very reliable, indeed.

The dd command is quite reliable. It does exactly what you tell it to
do, not what you think you told it to do. Had you given it a count of
sectors to read (calculated to match exactly the size of the ISO), it
would have read only those sectors. E.g.:

dd bs= count= if=/dev/dvd of=dvd1.iso

The CD burning software pads the ISO with blank data to round its size
up to a multiple of sectors. (I'm not sure why; perhaps someone more
familiar with CDs can explain this.) So what you got back was larger
than the original ISO. So of course the MD5 sums were different.

However, giving dd an exact count of sectors would not have
helped. Apparently CD burning software also mucks with a field in the
CD header as it burns the CD/DVD. So I would say that dd is quite
reliable, it just wasn't appropriate for what you wanted to do.

-- 

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