Actually, you can overwrite your hard drive 3 times with either FF or 00 to every sector and no info can ever be retrieved. A sector contains 256 words consisting of 32 bytes by I think nowadays 64 bits. If you write a 64 bit hexadecimal 0000 0000 or FFFF FFFF 16 times you end up with every sector containing nothing more than FFFF FFFF FFFF FFFF or 0000 0000 0000 0000 on the entire disc. Do it nough times and no geek no matter how smart he thinks he is, can ever pull anything else off of it. .
We're actually talking magnetic media. If you continually fill it with one character enough times, it will magnetize the media into whatever character you want. most popular with the gubmint in thier 'puters id is either FF or 00.
Hope that makes it as clear as mud. I may have some things backwards in this "loose" translation, as it has been awhile since i delved, but you get the idea, I hope.