[SLL] How do you Destroy a Hard drive

[Jesse Gordon]

----- Original Message ----- 
From: "Chuck Wolber" <chuckw at quantumlinux.com>
To: "Nicholas Bodley" <nbodley at speakeasy.net>
Cc: "SLL" <linux-list at ssc.com>; <jcs at snowandsnow.us>
Sent: Tuesday, August 01, 2006 2:18 PM
Subject: Re: [SLL] How do you Destroy a Hard drive


>
> Hmmmm, what's an "open-loop"?
>
> ..Chuck..
>

Open loop refers to a positioning system where purely mechanical means
such as a stepper motor & ball-screw or rack and pinion are used to position
the read/write head. No actual position feedback is used; but the 
controlling
system just assumes that the head is on track #48 because it's moved the 
stepper
motor 48 steps from position zero. (And it usually knows where position zero 
is
by either stepping so many steps in that direction that it hits end of 
travel
and therefore must be at zero, or it has a switch which indicates end of 
travel.)

If the stepper motor stalls for a step and gets one or more steps off, the 
controlling
system won't have a clue why it's not reading the data it wants. (That's why 
old floppy
drives and hard drives made such a racket trying to read a bad disk --  
constantly seeking to
sector 0 to re-assure that they correctly positioning the head.)

Closed loop means there is an independent position feedback system which 
informs the
controlling system of the head position at all times, regardless of whatever 
is going
on with the actuator. All modern hard drives and optical drives use closed 
loop -- usually
reading ID codes on the data tracks themselves to determine which 
cylinder/track they are over.

This is partly because modern storage types have just too many tracks per 
inch to reliably
and blindly position the read/write head, and partly because they don't use 
stepper motors any more.
They use either DC motors or voice coil actuators (sort of like a speaker 
coil) and of course
voice coils don't have any inherent position quantization like a stepper 
motor -- so some other
method of feedback MUST be used.

-Jesse