"M. Adam Davis" wrote:
>
> I doubt they place any of the drive parameters on the magnetic media
> itself.

Depends on the maker. They certainly DO put bad block substitution lists
on the media.

>  Should be located in a flash or eeprom.  The plates do,

It's cheaper and simpler to put it on the media. One less chip to
worry about. S/Ns are usually in the EEPROM.

> however, contain one or more servo tracks.  The most recent
> high-capacity drives actually contain one servo track for each regular
> track, and the head has two sensors, one that sits on the servo track
> and another that reads the data track.

The last 10 years worth of drives uses ONE head and use something
called "embedded servo" where servo information is stored between
the sectors. The second head you see is a data head, and improves
the seek time/capacity of a drive.

> The servo track helps improve seek time (the head can slam around a bit
> faster) and provides the feedback necessary to stay ona track once the
> head gets there - the voice coil is then in a closed loop mode.

The VC is almost always closed loop since the mechanical system
well characterized. Many drives 'read on arrival' and fill their
buffer as soon as they see a sector header for the correct track.
This again improves response times.

> It would take a lot of external magnetic field to change the information
> on the platter.

The flux density required is HUGE. You'll blow the electronics before
you get enough flux to erase the bits. And any airport scanner
that strong would be lawsuited into oblivion.

> But if you blew more than a few servo tracks then the

If you blew ANY servo tracks, the drive would stop working.
There is no easy way to get the needed track following accuracy
without embedded servo info. Better failed save, then have data
written that cannot be recovered.

> drive would probably run out of 'extra' replacement storage space and
> start reporting errors if it didn't cease to function alltogether.

This is what S.M.A.R.T monitoring is about. The controller reports
on the number of errors it sees, and when they start to rise, you
know that the drive is nearly failure.

> As a note, the click of death experienced by many Zip disk users was
> traced to servo tracks being inadvertently overwritten on the media.

Great firmware. And great example of how NOT to handle your customers.
"There is NO problem with Zip drives", even though thousands are
complaining. In no time at all people stopped buying drives, and
wouldn't buy them again once the firmware was 'fixed'. Burn me
once, shame on you. No chance will I give you to burn me twice.

> The head would continue to look for the servo tracks...
> click...click...click...

Robert


>
> -Adam
>
> Liam O'Hagan wrote:
>
> >Don't hard drives have some sort of onboard controller, which may be
> >scrambled by the scanner?
> >
> >>-----Original Message-----
> >>From: Jake Anderson [SMTP:grooveee@OPTUSHOME.COM.AU]
> >>Sent: Monday, February 09, 2004 1:45 PM
> >>To:   PICLIST@MITVMA.MIT.EDU
> >>Subject:      Re: [EE:] What are acceptable Hard Disk shipping packaging
> >>standards?
> >>
> >>the magnetic scanner really shouldn't do any harm. it might hash the drive
> >>contents but the only real danger would be induced currents in the drive
> >>PCB
> >>and they really should be minimal.

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body