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From: c60b-4ah@e260-2b.berkeley.edu (Phantom)
Newsgroups: comp.sys.mac.hardware
Subject: Re: Reformatted HD Disks
Message-ID: <1990Sep24.001125.9249@agate.berkeley.edu>
Date: 24 Sep 90 00:11:25 GMT
References: <369@shrike.AUSTIN.LOCKHEED.COM>
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If you stick a opaque tape on you High Density disk so that the hole that 
distinguishes it form a 800k disk is covered, it will probably work on a
FDHD (for a short while).  

That hole is used by FDHD to identify the density the disk in it.  
It is a useful feature not only because it enables you to format a disk 
without having to tell the computer the density of the disk every time, 
which is something you have to do on a PS/2, but also because it prevents 
people from accidentaly format a DD disk in HD format or the other way 
around.  Because the surface coating of HD and DD disks have quite different 
physical properties, one should refrain from cross-formating them.  Time and 
chance are all it takes your data to vanish if you insist on doing so.