Re: Bad Clusters

On Sun, 14 Aug 2005 10:09:01 -0700, "Robert Reader"

>Thanks for the detailed answer. I have two external Maxtor One touch hard
>drives I use for backup. I bought them 17 months ago and I ran the chkdsk
>just to make sure they were holding out OK. BOTH drives issued a "bad
>clusters" notice on one (not the same) file. I hope this doesn't mean they
>have both decided to self-destruct simultaneously.

I'd replace them, if they're still under warranty. The only exception
would be if you'd imaged from a failing drive to this one, and thus
possibly carried over now-irrelevant bad cluster markers.

>I sure wish someone would create a utility that would give you a clear
>picture of what's going on with the drive - whether its starting to cascade
>into failure or whether a bad sector was just a solitary, lone occurance.

This starts to look like the Halting Problem ;-)

If you knew enough to reach the answer, you'd have reached the answer.

>Like you said, I don't know if dozens of other sectors have recently been
>marked out as bad, which would suggest the catastrophic scenario.

Hard drives don't always die in a linear fashion, a few bad sectors at
a time. So catastrophic failure could always happen - e.g. if a logic
board fries, or a motor burns out.

However, progressive surface failure, either actual or apparent (e.g.
head positioning wobble that reduces accuracy of pickup, or a head
that doesn't work as well as it used to and thus can only read the
strongest of signals) is a common failure pattern.

Why would a previously good HD develop a bad sector?

Either because the ability to read the surface has generally
deteriorated, so that the weakest readable sector is now the strongest
non-readable sector (i.e. the first dead swallow of winter) or
something more focal has gone wrong with that particular part of the
drive. Which of these scenarios is the most comforting?

Well, if it's the first, then that's very un-comforting. Whatever
progressed to make this weakest sector no longer readable, will likely
progress to make other sectors unreadable too.

If the second, then; what could go wrong in one part of a disk?

The only thing that really comes to mind is what is called a "head
crash", i.e. where the head touches the disk and thus damages it.

Why would the head touch the disk - or perhaps, aren't the heads
always touching the disk anyway?

Unlike diskette drives, the heads of a hard drive are not supposed to
actually touch the data-bearing parts of the disk. They are supposed
to fly very close to the disk, riding on the cushion of air that
"sticks" to the surface of the disk as it spins - something that
actually works, when the disk is spinning at proper speed.

I can think of three things that can go wrong here.

Firstly, the disk could get bumped while it's active, causing the
heads to strike the disk in much the same way that a record would skip
tracks when you kick the turntable (as DJs and pensioners will know).

Secondly, the gap between disk and head is so small, that even a
particle of cigarette smoke may be big enough to have the same effect
as hitting a boulder on the highway. Head hits the object, and
bounces DING Ding ding on the disk; that could be today's bad cluster
and tomorrow's extension of this initial dead zone.

Thirdly, the disk might slow down due to a power drop, causing the
heads to pinch through the air cushion and touch the disk - and as the
disk is still likely to be moving apace, that's gonna hurt.

So we have scenarios that either cause physical damage, or are caused
by impurities within the supposedly-clean sealed airspace. In fact,
the one is likely to cause the other; a physical head strike may kick
up debris that from then on may swirl around in this now-polluted
airspace, and possibly cause further collisions and abrasion.

If the airspace is not polluted from head strike debris, why would it
be polluted? The most likely cause would be a failure of the air
filtering, such that the airspace is not so "sealed" after all.

When the heads hit the disk, is it only that part of the disk that is
damaged? Or are the heads also damaged, and thus left working
slightly less well than before?

Considering all this, how would you postulate a scenario of "just one
bad sector" without an increased likelyhood of more to follow?

The only one I can think of would be an absence of airspace pollution
from a failing air filter etc., plus an isolated head strike that
damaged the disk enough to cause a bad sector, but did not damage the
head enough to weaken its ability to function, plus the strike did not
kick up any loose debris, or leave the damaged surface unsmooth enough
to snag the head again. That's asking for a LOT of faith, IMO.



>---------- ----- ---- --- -- - - - -
Failure is not an option.
It's built into the software
>---------- ----- ---- --- -- - - - -
.

Relevant Pages

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