Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!thumper!ulysses!ucbvax!unisoft!gethen!farren
From: farren@gethen.UUCP (Michael J. Farren)
Newsgroups: comp.sys.ibm.pc
Subject: Re: RESET BUTTONS AND HARD DISKS
Message-ID: <682@gethen.UUCP>
Date: 12 Feb 88 11:15:28 GMT
References: <3505@batcomputer.tn.cornell.edu> <-497141@cpedev> <662@gethen.UUCP> <19824@bu-cs.BU.EDU>
Reply-To: farren@gethen.UUCP (Michael J. Farren)
Organization: There's Unix there in Oakland
Lines: 42

In article <19824@bu-cs.BU.EDU> madd@bu-it.bu.edu (Jim Frost) writes:
>This isn't necessarily true.  In all but the cheapest (and the very
>very expensive) drives, the heads will not touch the surface of the
>disk unless the disk is moves (relatively) violently.  This is why you
>usually need to park the heads only when you move the drive.

This isn't true.  One of the things that was added to drives with the
advent of Winchester technology was heads which can be landed on the
disk during a power-down.  With the exception of only those drives
which physically retract and lock the heads on power-down, ALL Winchester
drives land the heads on the media itself when the disk stops spinning,
and their design ensures that damage is minimal.  You park the heads
specifically to allow them to land on an area of the disk which does
not have data written on it, rather than taking the small chance of
damage.

>If the heads were only electronically suspended, they would lose suspension
>far before the drive stopped spinning.  The heads would scrape across the
>surface of the disk, damaging both the heads and the disk, until the
>disk came to a full stop.  Trust me, you'd hear it.

Nope.  The heads are aerodynamically suspended - they ride on a cushion
of air that is formed as the disks spin.  When the disks spin down, the
cushion disappears, and the heads DO land on the disk.  Their design,
however, is such that the damage to the head, and the disk, is minimal.
More expensive drives, like the Priam V70, actually retract the heads
before they land, but this is NOT the norm for inexpensive disks.

>I recall a time back in the CP/M days when a drive that just dropped
>the heads was turned off before doing a power down sequence.  The
>noise that that thing made was enough to make anyone cry.

Older disk drives were not designed to allow the heads to land - a
head crash was literally that, and usually destroyed heads and disks.
There is, or was, a pretty lucrative industry which specialized in
supplying replacement head assemblys.

-- 
Michael J. Farren             | "INVESTIGATE your point of view, don't just 
{ucbvax, uunet, hoptoad}!     | dogmatize it!  Reflect on it and re-evaluate
        unisoft!gethen!farren | it.  You may want to change your mind someday."
gethen!farren@lll-winken.llnl.gov ----- Tom Reingold, from alt.flame