[CoCo] 720kb vs 1.4mb 3.5" disks

[Gene Heskett]

On Wednesday 10 December 2003 17:36, Ray Watts wrote:
>Many years ago Marty Goodman wrote an article comparing the magnetic
>field intensity required to write to the different disks available
> to a CoCo user.  He recommended against using 3.5" HD (1.4mb) disks
> on the CoCo in place of the 720k DD ones.  His reasoning (if I
> recall it correctly) was that the HD disks were written to at a
> higher intensity, and therefore, if written to at the lower 720k
> intensity, would not reliably retain the data.
>
This is correct, using an HD diskette, written at the write currents 
that are correct for a DD diskette, will not saturate the coating 
magneticly, and you get a weak recording that may even have some 
remnants of the old data around the edges of the track from the 
incomplete erasure.  It will work for a while in the same drive, but 
probably will be pretty error laden when reading it in a different 
drive.  The weak fields also fade at a rate that can render them 
unreadable 2 weeks or less later.

I've been bit enough times by that, not realizing that when a 1.2 meg 
5.25" mitsumi has been slowed down to make a 720ker out of it, it 
still needs diskettes rated as QD in order to make a reliable 
recording.  Normal HD or DD 5.25" disks will be rather severely 
over-written in that drive, causing a partial erasure from the 
overdrive, and the effect to the user is the same, not very 
dependable storage. ITSR the last time I forgot and put a Dd rated 
disk in that drive, the verify phase marked over half of it out of 
the allocation table as unusable.

I have several 3.5" drives here too, and find I must use only DD 
diskettes in them on the coco else the verify phase suffers wholesale 
failures for taped up HD's, or even fails outright during the format 
for some drives, which demand a 500 kilobaud data rate if they find 
an HD hole in the diskette.  Those drives will work on the coco just 
fine with a DD diskette in them, but put an HD in and format fails on 
the first track.

>I have been using 3.5" 720k disks on my CoCo's for over 12 years,
> with bootable disks for over 11 years.  Occasionally, such as the
> old AOL givaways, I have possessed 1.4 mb disks.  Remembering
> Marty's article, I never used these disks for anything serious,
> just reformatted them to 720k for use as "scratch" disks.  About
> six years ago I began downloading RTSI files onto HD disks. 
> Neither the scratch disks nor the RTSI disks have ever lost a file
> and I am beginning to feel that Marty's warnings were overblown. 
> My rationale for this is that the HD disks probably have a superior
> oxide coating and will accept the lower magnetizing intensity at
> the slower rotational speed much better than anticipated.

One could blame your good luck on AOL buying the cheapest disk they 
could find.  After all, they knew 99.9% of them would just get 
formatted anyway. :-)

>Have any posters experienced problems using 1.4mb disks on a CoCo
>formatted to 720k?  If so, was there any evidence of the problem
> being due to the magnetizing force?

Lots of it, Ray

-- 
Cheers, Gene
AMD K6-III at 500mhz 320M
Athlon1600XP at 1400mhz  512M
99.22% setiathome rank, not too shabby for a WV hillbilly
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Copyright 2003 by Maurice Eugene Heskett, all rights reserved.