From: lasner@watsun.cc.columbia.edu (Charles Lasner)
Subject: Re: Disk and Drive incompatible....
Keywords: Disk 3.5 5.25 Formatting
Reply-To: lasner@watsun.cc.columbia.edu (Charles Lasner)
Organization: Columbia University
Date: Fri, 10 Apr 1992 07:19:54 GMT

In article <1992Apr10.060630.15107@u.washington.edu> pollock@milton.u.washington.edu (Joe Pollock) writes:
>In article <1992Apr9.233422.22209@cunixf.cc.columbia.edu> lasner@watsun.cc.columbia.edu (Charles Lasner) writes:
>>
>>I have never seen any problems with 3.5" disks, but as Mike said only with 
>>5.25" it doesn't work at all.  If the HD drive wrote on it there is no
>>problem, but when the LD drive writes or formats it, it becomes quite
>>unreadable.  Apparently the media saturates or is otherwise overloaded by
>>the greater writing current in the LD drives.  Never got an error-free disk
>>format this way.
>>
>>cjl
>
>My understanding is that there are a couple of factors involved here.
>The 5.25" HD disk is a higher permeability magnetic oxide than LD,
>which means it takes a greater magnetic energy to magnetize the 
>oxide.  LD drives vary in their ability to deliver here - but the
>situation is marginal at best.  The result is unreliability.
>
>In addition, the HD head writes a narrower track.  It cannot erase
>the wider track created by an LD head, so writing to an LD disk
>formatted on an LD drive - and probably containing data written by
>an LD drive - is also unreliable, especially when read on an LD
>drive.  Since the HD track is narrower, it may leave data in the
>"guard band" area between tracks, making the next read unreliable
>on an HD drive, as well.

Why?  Of course the "mixed" data situation is really bad on an LD drive, but \
since the HD drive only looks where it does, why would it matter for HD usage
exclusively?

No argument for LD usage later, but I never have problems with HD usage
of the media using LD-type format regardless of prior usage.  In fact, it's 
this insidious behavior that causes the other problem.  the user doesn't
see the problem because it only shows up on a disk that has been written
on an LD drive, then written on an HD drive then brought back to a LD drive
in that order.  The HD drive can always read the LD-written data, or the HD-
written data.

A more serious problem is registration drift due to hub rings on LD media,
as opposed to "pretend" media which is HD formatted as LD which is the
problematioc case to write on LD drives, as pointed out earlier.  This is
why Maxell MD1DD (not MD1D or MD2D) has no hub rings, because it is for
"quad" density stuff (250 KHz but 80 tracks) not LD or HD.  This is the
so-called 720K 5.25" format requested.  There are PD drivers for this format
available so AT's can read it.

>
>Using a 5.25" HD drive to format and write an LD disk usually works
>quite well, if the objective is to transfer data to a computer with
>only LD drives.
>
>BTW - I have taken disks badly trashed by various combinations of
>the above, run them through a bulk tape demagnetizer, and ended 
>up with error-free media (when used in the appropriate drives) when
>FORMAT reported 200K+ in bad blocks.
>
>When your campus has all combinations of floppy drives, and the
>bookstore stocks four different types of disk, and the clerks know
>no more than the users... :-(
>
>Anyone remember the old days of the 720K 5.25" disks, and problems
>with "Elephant" brand disks?

cjl