Re: secure erasure of files?

["Martin J. Bligh" <Martin.Bligh@us.ibm.com>]

> Some success is rumored to be able to be achieved by sampling the
> normal signal, and then subtracting the "expected signal assuming the
> current sequence of bits that was read". That way you might be able to
> recover the information that peeks out from below. 

It's more than rumour - I've seen this done. Dr Solomon's (whatever
they called their data recovery branch), early 1990's, England.
Maybe it was easier on older hardware like the MFM / RLL disks,
and certainly easier to piece together fragmented data with earlier
file formats.

I believe the point of overwriting 3 times (or whatever) is to reduce
the "subtracted difference" to noise levels where it's no longer useful.

> In practise all this doesn't work: The head will not be mispositioned
> 0.1 track to the same side during the whole revolution. Thus you will
> have parts of the previous data generation peeking out on the left
> side for part of the track and data from the generation before on the
> other side. Which you will see is not predetermined.

This only deals with your first method (which I agree, sounds unlikely
to work).

M.

PS. I've also seen a disk arm being wound across an opened disk
platter by a micrometer strapped to the head by a rubber band, 
to recover real data. Most amusing ;-).