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From: tjr@cbnewsc.att.com (thomas.j.roberts)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: 360K in 1.2M drives
Message-ID: <1991Mar1.155859.16874@cbnewsc.att.com>
Date: 1 Mar 91 15:58:59 GMT
References: <jcmorris.667836478@mwunix.mitre.org>
Organization: AT&T Bell Laboratories
Lines: 59

From article <jcmorris.667836478@mwunix.mitre.org>, by jcmorris@mwunix.mitre.org (Joe Morris):
> s64421@zeus.usq.EDU.AU (house ron) writes:
> 
>>I have often read that formatting and writing 360K drives in 1.2M 5.25"
>>drives is unreliable.  I have now obtained such a drive, and tried it,
>>and it seems to work perfectly every time (even after trying the disks
>>on genuine 360K drives).  Have the 1.2M drives got better lately, or
>>am I just lucky?
> 
> Lucky.  Whether or not you have a problem depends on both the drive you
> write the disk on, and the drive you read it on...especially the latter.

Yes, and no.

The real key, when writing 360K diskettes on a 1.2 Meg drive is:

	HAS THIS DISKETTE EVER BEEN WRITTEN ON BY A 360K DRIVE,
	OR BY A 1.2 Meg drive as a 1.2 Meg DISKETTE ??
	If the answer is yes, DO NOT use this particular diskette
	as a 1.2 Meg to 360 Kb transfer diskette, use another.

If you follow the following simple rule, you will (probably) have no
trouble at all transfering files from a 1.2 Meg drive to a 360 Kb drive:

	Format a blank diskette as 360 Kb on the 1.2 Meg drive.
	NEVER, NEVER, NEVER write on the diskette with anything
	byt a 1.2 Meg drive. (You can read it on either a 360 Kb
	or a 1.2 Meg drive).

Use another (360 Kb) diskette to transfer from 360 Kb to 1.2 Meg drives
(format it on the 360 Kb drive).


The reason for this rule is simple geometry of the read/write heads
of the two drive types: the 360 Kb drive has a head that is almost
twice as wide as the head on the 1.2 meg drive. When the 1.2 Meg
drive formats a diskette at 360 Kb, it writes EVERY OTHER TRACK
(i.e. at 40 Tracks/inch, instead of its own 80 tracks/inch),
with the appropriate bit density. Thus, as long as the space between
those formatted tracks HAS NEVER been written, a 360 Kb drive can
read the diskette, because there usually is enough signal to drive
the head, and there is no erroneous (old) signal to confuse it.
If a 360 Kb drive has EVER written to the diskette, then the space
between the (360 Kb on 1.2 Meg drive) formatted tracks will contain data
that is different from the desired data, and will sometimes cause data
errors.  The same applies if a 1.2 Meg drive had EVER written to the
diskette as a 1.2 Meg diskette.

	In principle, a 360 Kb diskette formatted on a 1.2 Meg
	drive does NOT conform to the specifications of a 360 Kb
	drive; and is not guaranteed to be readable on the 360 K drive.
	In practice, it is close enough for most 360 K drives, and
	works quite well.

Following the above rule I have transferred thousands of files between
1.2 Meg drives and 360 Kb drives, without a single error.

Tom Roberts
att!ihlpl!tjrob  TJROB@IHLPL.ATT.COM