HD Erase questions

[Prince]

I know how basic HD driver work. how FAT / BOOT sector work. I know how undelete works. What i want to know is, how can u unerase after the HD has been completely written 1 and 0 with some software? why its is common to write 1 and 0 , 3 times? (this question also relates to , how can u unformat after completely formatting?)

 

[QTPie]

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