Yeah, an existing magnetic field cannot be "killed", the flux is there and needs to
go somewhere. But it can be redirected through a path that is more "permeable"
to the flux than another path.
It is interesting to see how often people try to find a flux shield that actually
blocks the flux.
Obviously, all of them have the same idea in mind: if we can block the flux
on one half of a wheel that has magnets mounted on it, we can make the
wheel turn itself because we can make the attraction or rejection of the magnets
drop away on that side while the rejection or attraction on the other side
of the wheel stays the same.
Just like the idea that we could make a wheel move perpetually if we could
shield one half of the wheel from gravity.
Unfortunately there is not really a way to produce that type of shielding,
that simply makes one "pole" of a magnet "go away".
There is "shielding" that allows flux to flow through a specific path,
and so we can plan where the flux path is and thereby cause the magnetic field
to no be "projected" outside of that path.
This is what happens in your hard drive: the iron (could be nickel or cobalt)
is a very good flux path and better than the air or the other metals used
(probably nonmagnetic metals like aluminium, copper, nonmagnetic steel alloy,
that type of thing) and so the magnetic field does not extend outside of that path.
And so you don't "feel" a magnetic field outside of the flux path.
Nothing is really "shielded".
If we collect rain water in a gutter and allow it to flow to a drain, are we "shielding"
our street from rainwater? Or are we allowing the water another, easier, flow path?
regards,
Koen