Re: deleting a hard drive

From: Moe Trin (ibuprofin_at_painkiller.example.tld)
Date: 09/15/04

  • Next message: David H. Lipman: "Re: deleting a hard drive" 
    Date: Wed, 15 Sep 2004 15:47:34 -0500

    In article <wOJ1d.4214$F75.3528@trndny01>, David H. Lipman wrote:
    >I trust the US DoD's new methodology.

    I'm not sure I would.

    >It basically describes writing a pattern such as 01010101 the writing its
    >complement 10101010

    The fallacy is that this assumes the ancient FM encoding scheme from the
    1970s - and NO disk has used that scheme in _decades_ Modern hard disks
    use RLL 2,7 encoding, that takes groups of two, three and four bits and
    encodes them into four, six, or eight POSSIBLE flux transitions. The
    scheme is:

    DATA MAGNETIC CHANGE DATA MAGNETIC CHANGE
     10 NTNN 11 TNNN
    000 NNNTNN 010 TNNTNN
    011 NNTNNN 0010 NNTNNTNN
    0011 NNNNTNNN

    Thus, the '01010101' bit pattern is encoded on the disk as:

    010 10 10 10 10 10 10 10 10
    TNNTNN NTNN NTNN NTNN NTNN NTNN NTNN NTNN NTNN

    while the '10101010' bit pattern is encoded on the disk as:

    10 10 10 10 10 10 10 10 10
    NTNN NTNN NTNN NTNN NTNN NTNN NTNN NTNN NTNN

    Now, removing the spaces I put in here so that you can see the transition
    patterns, you get:

    TNNTNNNTNNNTNNNTNNNTNNNTNNNTNNNTNNNTNN
    NTNNNTNNNTNNNTNNNTNNNTNNNTNNNTNNNTNN

    You should be viewing this in a fixed width font on your news tool. But
    this shows that the complementary data pattern does NOT result in a
    complementary pattern ON THE PLATTER. Here, I show an X where the data
    changes from 'transition' to 'no transition' (or vice versa), and a dash
    where there is no change.

    XX-X-X-X-X-X-X-X-X-X-X-X-X-X-X-X-X-X

    So, half the magnetic domains are unchanged. Note that if you were to
    map the domains (North verses South), this is only s _SHIFT_ of where
    the changes occur.

    >then another patter such as 11001100 and repeating this six times.

    11 0011 0011 0011 0011 0011 0011
    TNNN NNNNTNNN NNNNTNNN NNNNTNNN NNNNTNNN NNNNTNNN NNNNTNNN

    0011 0011 0011 0011 0011 0011 0011
    NNNNTNNN NNNNTNNN NNNNTNNN NNNNTNNN NNNNTNNN NNNNTNNN NNNNTNNN

    TNNNNNNNTNNNNNNNTNNNNNNNTNNNNNNNTNNNNNNNTNNNNNNNTNNN
    NNNNTNNNNNNNTNNNNNNNTNNNNNNNTNNNNNNNTNNNNNNNTNNNNNNNTNNN
    X---X---X---X---X---X---X---X---X---X---X---X---X---

    Now you may have a clue as to why this is for "Unclassified DoD Computer
    Hard Drives" only. I repeat. If you are going to store stuff on a disk
    that you don't want anyone to see, YOU MUST DESTROY THE PLATTERS TO
    PREVENT ANY POSSIBLE DATA RECOVERY.

            Old guy

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