Re: Macintosh wiping (but why)

On Fri, 01 Apr 2005 14:22:29 +0100, "Paul Sanderson (Business 
Fax)"@sandersonforensics.co.uk said:
(cleaning up the top-posting)

> > Even worse, if the next track over is *also* out-of-spec, you can end 
> > up accidentally aquiring the servo signal for the *wrong* track.  
> > Whoops. ;)

> Not so, servo tracks have position information encoded within them - the
> drive knows when it is at the correct servo track  

Yes, but it has to acquire signal, settle on target, and *then* it can read the
position information - at which point it notices that the encoded information
doesn't match where it was supposed to be, and you get a "seek error".  That
definitely qualifies as a "whoops" in my book.

Most drives support some form of "recalibrate", which basically means "We're
*lost*, slam the head *all* the way over to one side and hope that Cyl 0 is
locatable at least", specifically for dealing with this sort of issue.  And
I've had failing drives that were able to locate a cylinder but only if the
seek was less than some small number of cylinders, and I've had failing drives
that could only locate the cylinder if it was seeking from Cyl 0 (data recovery
in these cases consisted of cylinder-sized 'dd' commands interspersed with
dummy commands to cause head seeks to the right places. Gaak ;)

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