Hi Stephen,
On Sunday 27 November 2005 22:32, Stephen Bird wrote:
> On Sun, 27 Nov 2005 14:25:37 +0000, Derrick wrote:
> >Not quite !!
>
> Agreed. :-) It was a bit of apples and oranges I was using.
>
> >The manufacturers utility is able to relocate bad blocks by
> > "sparing" to hidden cylinders. Which other tools cannot do ! But
> > as you say the end result is essentially the same.
>
> I didn't know "bad blocks" could be relocated thinking they were an
> actual physical location and I didn't know about "hidden cylinders".
> I still think it is magical how hard drives work. Amazing!
Bad blocks cannot be moved at all. Physical damage is usually the cause
of bad blocks. Not many people realise that most all hard disk drives
have the odd bad block or two. This is part of normal manufacturing.
What happens is this. When a drive is made, a number of cylinders are
reserved for use by the drive itself. These cylinders cannot be
accessed by anything other than the control system that resides in the
drive electronics.
When the drive detects a bad block it is able to add the block number to
a "Bad block Table" so that it can never again be used to store data.
When the number of bad blocks reaches a set limit, the lost data area
is replaced by using a spare cylinder. In this way there is no
apparent loss of storage size.
This mechanism is called sparing. The manufacturers hard disk utilities
are able to instruct the on board electronics to test every "Bit"
including spare cylinders, and re-write/refresh the bad block table.
A side effect of this, is of course, that any data anywhere on the drive
gets destroyed. However you do end up with, what is essentially a
virgin drive.
--
Best Regards:
Derrick.
Pontefract Linux Users Group.
plug at play-net.co.uk
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