Martin Brown wrote:
On 28/02/2012 20:25, Bernard Peek wrote:
On 26/02/12 22:08, The Natural Philosopher wrote:
Bernard Peek wrote:

If a change makes no difference to anything, ipso facto, it is not a
change. All changes therefore must make a difference, and are therefore
detectable.

Yes, but as I pointed out in the post to which you replied absence of
evidence is not evidence of absence. You can know that you haven't
detected a change, but you can't know that there is no change. Absence
of a change is not detectable.

Simple example would be waving a neodymium magnet near the computer but
not close enough to actually corrupt any data but close enough to alter
the magnetic domain strengths stored on the platter. There is a change
to the system but it is not detectable by standard drive electronics.

It is.

It would almost certainly result in several parity errors and 'bad blocks'


But for all practical purposes with a PC

Who said anything about a PC?

if the bitwise comparison of
the media with a standard reference copy is identical (including all the
bits not normally accessible) then it is unchanged.

I used to degauss my bank cards magnetic stripes with monotonous
regularity when I worked with powerful magnets.