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When wiping data with some programs you get different levels of 'security', i.e. the number of times they overwrite the data.

A basic level may be just overwriting it with 0's or random binary numbers, with a more secure solution being to do this 3 times (the standard) or an even more secure solution being to overwrite it up to 35 times.

Why would you have to do it more than just once? Surely when the code is changed it can never be taken back, so what's the need for overwriting it repeatedly?

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up vote 7 down vote accepted

There was a theory postulated years ago that it would be possible to read the original data because the head might drift slightly during the second write. Of course as you know, hard drives have become much denser since that time, making detecting the drift more difficult.

Regardless of the feasibility of that theory (it's never been done, just postulated), by the time you've rewritten it three times, the probability of picking up meaningful data is near zero.

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Because magnetic media is effectively storing binary data (bits) in an analog medium (the magnetic state of multiple pieces of iron oxide) AND the overwriting process may not entirely remove all traces of the previous state of that media, it is theoretically possible to infer the previous state of the data through forensic analyses. That said, three times with random data is more than enough to remove those traces unless your expected adversary is a well funded and very determined government.

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so on an ssd one wipe is bullet proof ? –  sam Nov 26 '12 at 22:24
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SSDs move new writes around for wear levelling so overwriting a file likely doesn't actually do so for the physical block. Not until the block is cleared so new information can be written to it will the old data be physically erased. For performance reasons clearing blocks can be delayed if reads need to be serviced and there is already enough cleared blocks to handle current writes. –  Brian Nov 26 '12 at 22:36
    
@sam there'a no good way to make sure you securely erased a ssd. –  Alex Nov 26 '12 at 22:59
    
@Alex - Yes...Yes there is. The simple process encrypt the sdd out of the box, place data on it, then issue a command to dump the data using drive's ssd tools normally provided. The result is encrypted data that cannot be retrieved since its random data. All the tool does is marks the data for deletion. Since its encrypted it cannot be undeleted, because again, it srandom data. –  Ramhound Nov 26 '12 at 23:31
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@Ramhound encrypted data, not random. –  Alex Nov 27 '12 at 14:24
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