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From: winders@aux.support.apple.com (Scott Winders)
Newsgroups: comp.sys.mac.system
Subject: Re: HD disks failing at 800K format
Message-ID: <51425@apple.Apple.COM>
Date: 11 Apr 91 18:44:48 GMT
Sender: nntp@Apple.COM
Organization: Apple Computer, Inc.
Lines: 38

In article <1991Apr10.204450.8125@news.nd.edu> 
rcook@ircsun1.ccmblab.nd.edu (Robert Kelley Cook) writes:

> In article <1991Apr10.160835.5153@dartvax.dartmouth.edu>, 
Charles.E.Dubuque@dartmouth.edu (Charles E. Dubuque) writes:
>> 
>> Since HD disks have to store about 2 times as much information in the
>> same surface area, I would imagine that: 1) the HD drive writes a
>> signal that is of lesser strength than an 800K disk, otherwise it would
>> destroy data. 2) that the HD disks are of a more sensitive medium than
>> 800K disks, since it needs that extra sensitivity to pick up the weaker
>> signal.
>> 
> . . . 
>> 
>> All this adds up to the problems you describe.
>> 
>> Moral of the story: never use HD disks in 800K drives. It isn't meant
>> to work that way (notice that Apple doesn't give you the option of
>> formatting HD disks ar 400K,800K or 1.4M)
>> 
>> Chuck
> 
> 
> Actually, A high density disk needs a greater signal to flip the 
> particles, because the medium used is magnetically less susceptible to 
> spurious changes. They won't let you format high densitys as low density 
> disks, because a low density drive doesn't nessecarily have the oomph to 
> flip the bits of a high density disk even though it can read them.
> 
> Robert Kelley Cook
> U. of Notre Dame '91

Not true Robert, Chuck is right.  The coating on high density diskettes is thinner and has finer particles. This requires a less intense magnetic field from the read/write head to properly align the magnetic particles within the data cell during a write cycle. The magnetic field generated by Apple 800K and 400K drives is too strong and may cause data loss. 

Scott Winders
internet: winders@aux.support.apple.com
AppleLink: winders.s@applelink.apple.com