Data is kept on your hard drive until you overwrite it but I have never heard of needing to overwrite at least 30 times.Say you have a 4 gig hard drive and it is filled to the max.You delete all of it.It would still be on the drive until you overwrited it but if you had to overwrite it 30 times you would have way to much data that the drive couldnt hold.I couldnt explain that very well.