[wplug] degaussing tapes and hard drives

[Burt E Reany]

> For the most part it's not possible to erase these tapes and hard drives
by overwriting them.

   IMHO, I differ with this statement.  CPIO  hundreds of compressed copies
of the /bin and /kernel (non-user) directories to a tape, until you reach
EOT, and you'll drive the theoretical data recovery people sane.

  A.  We aren't trying to erase the tapes, but to make the data that we had
stored on them unrecoverable by others.

  B.  The degausser field differs in field strength across the face of the
tape, as it is beside the tape's edge rather than across it's face, as the
r/w heads are.  The field strength is constant at any track - closer tracks
will be effected by higher field strength than the farthest tracks are. The
constant field, tho, is the problem; it might weaken the residual bit-shift
flux on a track, but never changes its -polarity.  Higher-gain read heads
can often recover the data. Re-writing every character with a fixed pattern
of zeros can also leave enough residual signal for the higher-gain
equipment to recover.

  C.  Overwriting the tape with useless data applies the same signal
strength to each track as the original data was recorded at, and applies so
many new  field shifts that the originals can no longer be isolated - if
the old ones were still detectable, you couldn't read the new data back in
from the tape.

  D.  Inter-record gaps might still expose recoverable data, but
lengthening the physical blocksize,( and/or rewriting the tape with 2
passes, each of different blocksizes), will make any sort of automated
recovery most unlikely.

  E. Degaussing disks works equally badly, except the field strength across
an entire track remains constant, as all points are equidistant from the
coil, so data is more easily recovered from disk than tape

  F. Deleting files removes the blocks of data to the freelist, but doesn't
overwrite the data in these newly-freed blocks. The chaos theory of writing
useless trash until you fill the disk, however, does overwrite every block
of the freelist with garbage that makes recovery of older data
unreasonable.

  G.  Osama supports school prayer.






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