As one can find in the Disk Utility/Erase/Security Options section, there are two options for erasing disks securely - zero out 7 times or zero out 35 times. Where these numbers come from?
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The 7 and 35 passes very probably come from the paper "Secure Deletion of Data from Magnetic and Solid-State Memory" by Peter Gutmann. There, he described various overwrite patterns targeted at specific hard drive write encodings.
However, the paper, and the 35 passes, are now obsolete, as they were for old hard drive technology, as even the author readily admits. Nowadays, to delete a disk, a single pass with zeroes is enough. Even better, use the SECURE ERASE feature of all modern drives.
For more discussion see the Wikipedia article on the Gutmann method.
Neither of those should be considered the least bit secure. Just zero-ing out the data is not enough, no matter how many times you do it.
All so-called secure erase software should follow recommendations such as those given in Secure Deletion of Data from Magnetic and Solid-State Memory by Peter Gutmann. This paper recommends 35 write passes, some of which use random data and some of which use specific patterns, only one of which is all zeroes.
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