Why does mechanical failure causes HDD being undetectable by bios or OS ?

Hi!
Could someone please explain why in the case of *mechanical* failure HD
becomes sometimes undetected by BIOS and/or the operating system (e.g. win xp
or linux)?
If it was an electronic failure then such behaviour would be obious, but why
the same happens with some mechanical failures? When electronics is working in
my opinion it still should be detected by bios and/or the system (win xp or
linux), but often it is not.
I could recover about 80% of the data from my HDD (which apparently has a
mechanical failure - plates spin up and down, heads create bad noises) if only
the disk could be seen by the system all the time. But often during copying of
the data heads hit with a loud sound so badly that sometimes even the plates
stop rotating, and the disk then dissapears from the system. It is then very
difficult to make it detectable by the system again, sometimes the sytem can
detect it but only after several minutes of copying it freezes and then
dissapears again.
Recently, I was unlucky, and even after several dozens of retries it's still
undetectable by the system.

Could you please advice what to do to make the disk detectable by the system
all the time?
What causes that it is not detectable although the failure is in mechanics not
electronics?

BTW, if someone has the same disk model (Quantum Fireball ST64A011), please
let me know.

andy

On Fri, 03 Sep 2004 17:41:49 GMT, "CWatters"
wrote:
This sounds like a classic head crash. The heads are physically rubbing on
the platters causing damage to both. Onee that happens it's only a matter of
time before the drive is completly dead.

It still doesn't explain why head crashes causes disk to be undetectable at
times (well, most of the time in this case).
I opened the disk (I don't care about the dust, since it's dead anyway), and
the surface of the first plate is in perfect condition, I can't see surfaces
of other plates, though.

BTW freezing didn't help to detect it.

a.

"andy" wrote in message
...
On Fri, 03 Sep 2004 17:41:49 GMT, "CWatters"
wrote:
This sounds like a classic head crash. The heads are physically rubbing
on
the platters causing damage to both. Onee that happens it's only a matter
of
time before the drive is completly dead.

It still doesn't explain why head crashes causes disk to be undetectable
at
times (well, most of the time in this case).
I opened the disk (I don't care about the dust, since it's dead anyway),
and
the surface of the first plate is in perfect condition, I can't see
surfaces
of other plates, though.

BTW freezing didn't help to detect it.

a.

If the disk is not spinning at the required speed it is not READY, and
cannot be read. The system cannot read devices which do not decalare
themselves as READY!
Mike.

"andy" wrote in message
...
On Fri, 03 Sep 2004 17:41:49 GMT, "CWatters"
wrote:
This sounds like a classic head crash. The heads are physically rubbing
on
the platters causing damage to both. Onee that happens it's only a matter
of
time before the drive is completly dead.

It still doesn't explain why head crashes causes disk to be undetectable
at
times (well, most of the time in this case).
I opened the disk (I don't care about the dust, since it's dead anyway),

WACKO!

"andy" wrote in message
...
On Fri, 03 Sep 2004 17:41:49 GMT, "CWatters"
wrote:
This sounds like a classic head crash. The heads are physically rubbing
on
the platters causing damage to both. Onee that happens it's only a matter
of
time before the drive is completly dead.

It still doesn't explain why head crashes causes disk to be undetectable
at
times (well, most of the time in this case).
I opened the disk (I don't care about the dust, since it's dead anyway),
and
the surface of the first plate is in perfect condition,

You can't always see the damage with the human eye until it's really very
bad.