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From: bskendig@burn.Princeton.EDU (Brian Kendig)
Newsgroups: comp.sys.mac.system
Subject: Re: Low Density vs High Density
Message-ID: <6481@idunno.Princeton.EDU>
Date: 21 Feb 91 22:05:23 GMT
References: <1991Feb21.211235.11051@ux1.cso.uiuc.edu>
Sender: news@idunno.Princeton.EDU
Organization: Starfleet Academy: Princeton University
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In article <1991Feb21.211235.11051@ux1.cso.uiuc.edu> robs@ux1.cso.uiuc.edu (Rob Schaeffer) writes:
>Can a high density drive be made to read a high density disk
>that was formatted in a low density drive?

In a word: no.  A high-density disk formatted in a low-density drive
is unstable; even low-density drives other than the one it was
formatted in probably won't be able to read the disk.

Here's the reason as I understand it (and more knowledgeable souls may
want to throw in a correction here and there):

A high density disk crams much more magnetized information into the
same amount of space.  Therefore, the disk has to be made more
resistant to having its data changed, or else the disk will end up
scrambling itself!

A high density drive uses a more powerful electromagnetic field to
deal with the more resistant HD disks.

So:

If you format a high density disk in a low density drive, then in an
ideal world, it should either format fine, or not at all because the
low density drive field isn't strong enough to change the information.
But in the real (?) world, it turns out that the slight imperfections
in the disk and in the drive combine to affect some parts of the disk
but not others, so the disk may become weakly formatted.  Put this
disk into any other drive, and it's like trying to fit the wrong
puzzle-pieces together.

If you format a low density disk in a high density drive as high
density, then the disk will be treated with a much-too-powerful
electromagnetic field.  When you set one bit of information, you'll
probably end up erasing four or five nearby bits.

Note that high density drives can recognize and deal with low density
disks as low density; they can reduce their field strength.

I've also heard rumors that once you format a HD disk as DD (double
density, or low density -- the SD single density disks went out of
style years ago), you can never reformat it as HD again; the format
always fails Or something like that.  (Can anyone clarify this?)

And I'd assume that the 2.88M ED disks behave in a similar way to all
this.

I hope I've cleared things up somewhat.

     << Brian >>

| Brian S. Kendig      \ Macintosh |   Engineering,   | bskendig             |
| Computer Engineering |\ Thought  |  USS Enterprise  | @phoenix.Princeton.EDU
| Princeton University |_\ Police  | -= NCC-1701-D =- | @PUCC.BITNET         |
"It's not that I don't have the work to *do* -- I don't do the work I *have*."