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: