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From: nraoaoc@nmtsun.nmt.edu (Daniel Briggs)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Is there anyway to know a disk has been copied?
Message-ID: <4109@nmtsun.nmt.edu>
Date: 14 Apr 90 20:53:53 GMT
References: <2885@milton.acs.washington.edu> <3133@umbc3.UMBC.EDU> <1990Apr14.174605.18601@cunixf.cc.columbia.edu>
Reply-To: dbriggs@nrao.edu (Daniel Briggs)
Distribution: na
Organization: National Radio Astronomy Observatory, Socorro NM
Lines: 42

In article <2885@milton.acs.washington.edu> jimli@milton.acs.washington.edu (Jimmy Li) writes:
>I'd like to know if there is any way to know if a floppy disk has been copied.
>It doesn't seem to be possible to me. But a friend of mine insists that it
>could be done. 
>
>Any ideas?
>-Jimmy.

I think I may have heard some folk tales about what your friend is
driving at.  I have *no* idea if this is true, or even possible.  I
merely toss this out as an example of this kind of reasoning.

What you would need would be some sort of data on the disk that would
be destroyed by the process of reading.  (Say a bit that is on the
very marginal edge of validity, or a pattern of them designed to take
advantage of some physical property of the read/write head.)  This
data would be put somewhere on the disk where normal DOS calls
wouldn't get at it, (but possibly where a disk-to-disk copy would?)
Does anyone know if there are any intertrack areas that are not
normally accessed by normal sector reads, but would get scanned by a
disk copy?  Anyway, what's worse still is that you also need a program
that is capable of driving the disk hardware close enough the flag
data to see if it's still OK, but not close enough to zap it.  Sounds
very hardware dependant to say the least, a nightmare of calibration
probably, and impossible possibly.  I have no doubt that this kind of
stunt might be doable in Maxell's recording media development lab, but
it sounds awful damned iffy with off the shelf PC hardware.

Lest you think that I am completely off base, I point out that there
is a copy protection scheme that is *sort of* based on the same kind
of techniques.  (That is, using non-linear effects in the media.)
That's the one where the original disk drives write bits that are
smack in the middle of the indeterminate region between 1-and-0.  A
normal drive will read this back sometimes as 1 and sometimes as 0.
The software, of course, checks for this property.  When this disk is
copied, the destination drive writes a 1 or a 0, but not both.  I
always thought that this was kind of a neat trick.

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