If you were to simply overrite data with zeros it becomes irrecoverable (see 'The Great Zero Challenge').
CCleaner offers two methods of data deletion: "Normal" and "Secure".
It is to my understanding that "Normal" simply performs the afortementioned: overrites data with zeros.
However, what this "secure" method does is overrite said data with more randomly generated data afterward, making it extra irrecoverable. It even goes as far as to offer several different amounts of passes, making it extra super irrecoverable.
This is where I become confused: if overriting data with zeros (e.g. "Normal" file deletion) makes it irecoverable to begin with, why waste time and harddrive space passing over the deleted data several times over again with randomly generated data (e.g. "Secure" file deletion)?
