Re: HD 5 1/4 disks on Apple //?

[mjmahon@aol.com (Michael J. Mahon)]

Roger Johnstone wrote:

>In <20031227133637.04179.00002017@mb-m29.aol.com> Michael J. Mahon wrote:
>? 
>> 3.5" HD diskettes are very MARGINAL in Apple 800K drives, with some
>> not working at all and some working for a few hours or days.
>> 
>> 5.25" HD diskettes are ABSOLUTELY IMPOSSIBLE in a Disk ][,
>> because the media difference is much greater, and the Disk ][ cannot
>> generate sufficiently strong write/erase fields.
>> 
>> This must all be in the FAQs.
>> 
>> -michael
>
>I have formatted high-density 5.25" disks in an Apple II third-party 
>disk drive. As with the 3.5" disks it's a case of some brands of disks 
>working, and some not working at all. I only used them to try out some 
>software from downloaded disk images though, so I never tested their 
>reliablility.

I'm very curious about the operation of this third-party drive.

The coercivity of 5.25" HD media is about twice that of DD media,
so any drive that could write it reliably would be generating a write
field _way_ out of spec for DD media.  This could result in writing
disks that normal drives could not easily overwrite, or in corrupting
information written to the other side of the diskette.

The coercivity difference for 3.5" HD and DD media is only about
20%-30% different, so, with the "right" combination of things being
at the edges of their tolerances, it is possible to obtain useful disks,
but a factor of two is considerably outside the allowable range.

I had considered that the drive may be sensing the type of media
and adjusting its write current, but 5.25" diskettes do not have
a media sensing notch...

-michael

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