hard disk activity

[VSmirk]

Pretty much, yeah.  Except I need diffing a pair of files that exist on
opposite ends of a network, without causing the entire contents of the
file to be transferred over that network.

Now, I have the option of doing this:  If I am able to determine that
(for instance) bytes 10468 to 1473 in a 849308 byte file are the only
segment that has changed, I can send that range over the network and
insert it into the right place; and then, with a downtime overnight, I
can do a file-copy synchronization to ensure there were no errors
during the day.  (I'm reading this and wondering if it even makes
sense, sorry if it doesn't.)

But the trick in my mind is figuring out which specific bytes have been
written to disk.  That's why I was thinking device level.  Am I going
to have to work in C++ or Assembler for something like this?

Sorry if this sounds like a newbie question.  I've been working with
Python long enough to know that someone out there has already solved
one or another of a really obscure problem.  So I thought I'd take a
stab at it.

Thanks everyone for the great links. 

V