Write a few times. I suggest rewriting it 7 times. I remember that, it was the magic number.... From somewhere :-\.. And try different values on each pass.
"There are many methods, or standards. In US standard, the write head passes over each sector of Hard Drive 3 times. The first time with zeros, the second time with another value and the third with random characters. In German standard (in Germany data security is extremely considered), the write head passes 7 times."