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From: tchi@sal-sun9.usc.edu (The Answer Company)
Newsgroups: comp.sys.mac.system
Subject: Re: Low Density vs High Density
Message-ID: <15143@chaph.usc.edu>
Date: 22 Feb 91 00:25:52 GMT
References: <1991Feb21.211235.11051@ux1.cso.uiuc.edu> <6481@idunno.Princeton.EDU>
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In article <6481@idunno.Princeton.EDU>, bskendig@burn.Princeton.EDU (Brian Kendig) writes:
|> 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*."

Hmmm...  From what I remember, the magnetic field strengths are the opposite from what Brian mentioned (ie. the low density drives have a higher field strength than the high density drives).  The high density disks are more expensive because it is harder to make magnetic media that requires LESS field strength to write data to the disk.  Therefore if one puts a hole in the DD disk and try to pass it off as a HD disk in a high density drive, the high density drive does not have enough field strength to writ









e data to the disk so when you try to read the data the next time, boom!  I used to work at ASUCLA's graphic services where we rent time on the Macintosh.  I've had no problems with people using HD disks in low density drives (ie. Plus, old SE, and Mac II) as long as the disk is formatted in a low density drive (obviously) and they didn't use the disk in a high density drive.  Again, the HD drives can read data from a DD disk since the HD head is more sensitive (ie. needs less field strength to distinguish









 ones and zeros) but the data is written a too low of a field strength for the DD head to pick it up.  Maybe the HD drive recognized that a HD disk is in the drive and uses less power to write data and when a DD disk is in the drive, it steps up the juice to write to DD disks.

Anyways... I was told all this by some Apple Certified Techs at the ASUCLA Computer Store awhile back.  Hopefully what I've said (wrote) is correct.  My solution is to stick with DD disks and not worry about the lost 650K and so far I've had no problems using them in a IIcx and a SE/30.  Of course I only have a Mac Plus so I don't really have a choice... :)

Sorry the message is so verbose...

Tony Chi (tchi@usc.edu)
The Answer Company: typesetting, graphic design, Macintosh consulting

I'm getting my Masters in EE just for fun...