Date   : Sun, 06 Jun 2004 10:53:05 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: 3.5" disk drive

> Message-ID: <1086349807.18038.36.camel@...>
 
Jules Richardson <julesrichardsonuk@...> wrote:
> Presumably to be as reliable as possible, only a double density drive
> should be used with the beeb (rather than one capable of accessing high
 
No. All standard 3.5" drives look as though they are a double-density
drive to the BBC when used with double-density disks.
 
> density disks). Aren't the heads very slightly narrower on a HD drive,
> which *may* cause problems if you then use DD media written with a HD
> drive in another system? I may well be going mad there, though :-)
 
No, you're confusing that with 40 track and 80 track 5.25" drives. Don't
confuse data density (amount of data in a track) with track density (how
many tracks on a disk).
 
Old 40 track 5.25" drives used heads that are half the width of 80 track
5.25" drives. To make matters worse, later 40 track drives used a narrow
80-track head but had a large track step to access only 40 tracks. Also,
dual-stepping drives that could be switched between 40 track and 80 track
obviously had a narrow 80-track head in them.
 
The problem would occur if you formatted a 40-track disk on an old
40-track drive with a wide head, and then wrote data to it on a
narrow-head drive, such as a 40-track drive with a narrow head or an 80
track drive set to double-step. The data track written by a narrow head is
narrower than the data track written by the wide head:
 
            -------------------------------
wide head                 -----------------  narrow head
data track                -----------------  data track
            -------------------------------
 
The problems occur if you format a disk with a wide head, and then write
data onto the disk with a narrow head.  You can get bleed-through of data
from the area outside the narrow data track from the old wider data track
laid down by the wider head.
 
The best thing to do is ensure that you format disks with a narrow-head
drive, either a newer 40-track drive or an 80-track drive in 40-track
mode.
 
None of this applies to 3.5" drives. Only a small number of 3.5" drives
were 40-track, and the head they used was the same width as 80 track
drives.
 

-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
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