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From: derosa@cell.mot.COM (John DeRosa)
Newsgroups: comp.sys.mac
Subject: Re: Beware formating HD floppies in DSDD drives
Message-ID: <1612@diamond2.UUCP>
Date: 13 Mar 90 16:51:40 GMT
References: <1567@diamond2.UUCP> <1408@bridge2.ESD.3Com.COM>
Distribution: comp
Organization: Motorola Inc., Cellular Infrastructure Div., Arlington Heights, IL
Lines: 38

ngg@bridge2.ESD.3Com.COM (Norman Goodger) writes:

>In article <1567@diamond2.UUCP> derosa@cell.mot.COM (John DeRosa) writes:
>>.......BEWARE: If you now put this HD/800K hybrid floppy
>>in a HD Drive (se-30, IIX, IIcx, etc), the floppy
>>will be rejected with a "unrecognized disk, do you
>>want to format?" dialog box.  Upon review o.....

>.....When you
>format and use an HD diskette as an 800 or 400K disk, the FDHD cannot
>effectively overwrite the stonger signals that the previous formatting
>created.......

I did not know that the signals on a 800k and 400k disk are stronger than
on a HD disk but this would make sense.  The tracks must be closer together
therefore the signal must be lower to not cause interference between
adjacent tracks.  This does not hold for the 800/400 difference as the only
change was one sided vs. two sided.

The problem with your argument is that I did not want to initialize the
disk, just read it.  The dialog I get tells me that the drive  can
not read the 800K "HD" disk while it can read a true 800K disk.  This means
that the drive recognises the disk by mechanical means, i.e. the notch
extra "write protect" hole, instead of by sensing what is on the magnetic
surface of the disk.  The mechanical sensing must then change the bias
of the head to read HD only and the "stronger signals" of the 800K
confuses the reading of the data.  

I guess that it just comes down to a problem 
that is so obscure (who would ever waste an expensive HD disk as a DSDD?)
that apple/sony took the easier approach.  I'm an engineer, I can under
stand the tradeoffs. 
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=    John DeRosa, Motorola, Inc, Cellular Infrastructure Division           =
=                   e-mail: ...uunet!motcid!derosaj                         =
= I do not hold by employer responsible for any information in this message =
=      nor am I responsible for anything my employer may do or say.         =
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