Only putting all zero's will not work, to make the magnetic medium change for sure, it must change state so ones and zeros are used in random order or an algorythm is used. A zero for a null and a one for a pulse. The programs used, do this many times as many as 35 on some and as few as once on others. The government (defense department) uses three. Here is a much better explanation
http://www.cs.auckland.ac.nz/~pgut00...ecure_del.html
My "low level" format runs on a 160g drive in one and a half hours, the program for complete return to unused actually takes only 7 minutes longer. You can't actually change a magnetic image on the medium without an actual change in written matter. Many of the programs do run a complete zero pattern last and is sometimes called a zeroed out drive
or nul drive but that is actually not neccessary. Our company uses one of the programs that writes a five pattern algorythm and it is satisfied that the disk is good to go.