Stephen mentioned one aspect - impact if someone recreates the data.  
Another is the technical capability of the hard drive recipient and 
anyone else that gets the drive.  Overwriting with 0's (or 1's) creates 
a regular pattern that can be filtered out to retrieve the remnants of 
the previous data, which is why the DOD standard is 7 passes.  That 
being said, I've never met anyone who had the technical capability to 
retrieve data off drives once they've been overwritten.

As fate would have it, I've developing a CD/USB-bootable image whose 
sole purpose is DOD-wiping every drive in the system.  It's like DBAN 
(Derek's Boot and Nuke) on steroids.

As far as bs (block size), in my experience, bs affects the speed of the 
dd.  Too small or too large and the time increases.  For the ATA drives 
I've dealt with, the sweet spot was 32K, but this depends completely on 
drive type.  Clearly YMMV and you might experiment with different 
values.  Wrap the dd command in a for loop and time the execution of the 
dd command.  Create a 1GB test file and then run something like this:
dd if=/dev/zero of=/tmp/testfile bs=1024 count=1024000  # close enough
while [ $BS -le 134217728 ]; do
      COUNT=$((1048576000/$BS))
      echo BS=$BS
      dd if=/dev/zero of=/tmp/testfile bs=$BS count=$COUNT
      BS=$(($BS+$BS))
      echo "-------"
  done

When I ran this, I got speeds that varied by up to 50%.  Finding the 
right blocksize can save you several hours.

Regards,

George Toft

On 8/18/2013 8:19 PM, keith smith wrote:
>
> Hi All,
>
> I have an old computer that I am giving to a friend so I wanted to 
> wipe the drives in preparation for that.
>
> The master is 250GB
> The slave is 1TB.
>
> I read a couple articles that suggested using a rescue disk and the 
> shred utility to take care of this.  I also read that shred is not 
> necessary to just write all zero's to the drive.
>
> The rescue disk I am using is DVD disk one of CentOS 6.3.
>
> I ran shred on the fist drive.  It took 4.5 hours to run 3 shred 
> passes plus 1 that writes zeros to the entire drive.
>
> Command : shred -zv /dev/sda  (this was on the master disk)
>
> Then I ran : dd if=/dev/zero of=/dev/sda bs=16M
>
> In one of the articles it showed the above command with bs=1M
>
> Does the size of "bs" matter?
>
> Also what about the argument that shred is overkill?
>
> Thanks!!
>
> Keith
>
> ------------------------
> Keith Smith
>
>
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