Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!uw-beaver!ssc-vax!bcsaic!paula
From: paula@bcsaic.UUCP (Paul Allen)
Newsgroups: comp.sys.ibm.pc
Subject: Re: RESET BUTTONS AND HARD DISKS
Message-ID: <3922@bcsaic.UUCP>
Date: 13 Feb 88 07:45:36 GMT
References: <3505@batcomputer.tn.cornell.edu> <-497141@cpedev> <662@gethen.UUCP> <19824@bu-cs.BU.EDU>
Reply-To: paula@bcsaic.UUCP (Paul Allen)
Organization: Boeing Computer Services AI Center, Seattle
Lines: 72

In article <19824@bu-cs.BU.EDU> madd@bu-it.bu.edu (Jim Frost) writes:
>In article <662@gethen.UUCP> farren@gethen.UUCP (Michael J. Farren) writes:
>>>>I am told that a hardware reset button is a good idea if one has a hard 
>>>>disk. Why? 
>>
>>Because it allows you to do a complete cold boot without powering down
>>the disk drive.  If the disk is powered down, and it doesn't have auto-
>>parking (as most smaller disks do not), then the heads land wherever
>>they happen to be.  This will most likely not be a problem, but it could
>>be - you could lose data.  The reset button will allow the heads to keep
>>flying, thus never touching the surface, thus preventing this type of
>>data lossage.
>
>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.  Think of
>it this way: drives take several seconds to spin down.  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.

Excuse me?  Electronically suspended?  From other postings, I assume
that Jim knows how disks work, but perhaps some clarification is in
order here for those who really are curious why a reset button is
desirable.

The heads in Winchester drives are designed to rest on the disk surface
when the power is off.  When the disk is spinning, the heads are
suspended aerodynamically in the wind that is dragged along by the
platters.  Due to the Bernoulli effect, the heads approach the disk
surface very closely, but do not touch it.  When the drive is in the
process of spinning up or down, there is a period of time when there is
not enough wind to keep the heads flying.  During this time, the heads
slide on the surface of the platters.  Parking the heads moves them over
an area of the disk that doesn't have data on it.  Parking is advised
before a move because a shock can cause the heads to *bounce* on the
disk surface, resulting in loss of data.  Some early drives (like my
5.5M Shugart) land a little roughly on power down, and need to be parked
each time.  But by the time the XT came out, the drive makers had
figured out how to build drives that could be safely shut off without
parking.  Many of the drives that have come along since automatically
park the heads on power-off.  So, for just about any drive you're likely
to see in a PC-class machine, you generally won't lose data if you turn
the power off.

>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.

Sounds like a wonderful drive!  It has a head crash if you don't power
it down just right?  What kind of drive was that?  (I'm really curious.
I toyed with the idea of hacking a hard disk into my old CP/M system,
but never had the cash.)

>The best reason for the reset switch is that the drive doesn't
>have to be spun up again.  Spin up is the most strenuous time for a
>drive and also for the computer's power supply, and it is best to
>avoid doing it more than necessary.

THIS is  why a reset button is a nice thing to have!

>jim frost
>madd@bu-it.bu.edu

Paul

-- 
------------------------------------------------------------------------
Paul L. Allen                       | paula@boeing.com
Boeing Advanced Technology Center   | ...!uw-beaver!ssc-vax!bcsaic!paula