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How platter swelling affects a hard drive

  Okay, I know this is not about how to read bad parity in a drive in order to find a stale drive in a RAID five.  This is an important subject, however, and I also think it is important to know why heat and a swelling platter can cause hard drive damage.

  In its simplest explanation a hard drive works much like the old phonograph record players.  The record would be placed on a spindle and a needle that is on the end of a tone arm would be placed on the record.  The needle reads the recorded music from the record and transfers that data to the amplifying device.  Now that being said how is a hard drive like a phonograph record? 

  A hard drive has a platter or a set of platters that are similar to the vinyl record.  These platters are mounted on a spindle. There is also a ‘tone arm’ only on a hard disk it is called an actuator arm.  At the end of the actuator arm is a set of heads which are comparable to the phonograph needle.  This is a basic description of a hard drive, however, there is one huge difference in functionality.

  On a phonograph record the needle sits ON the record.  With a hard drive the needle floats on a cushion of air over the platters.  Since this is the technology used then it is important that the heads remain at a constant distance from the platters.  When a head reads a hard drive it doesn’t send a single beam to the hard drive to read it.  The signal looks like a cone and the farther the head gets from the drive the wider the cone is.  Conversely, the closer it is to the platter the more compressed the cone is. 

  Each track on a platter is basically equidistant from each other.  So, if the head is the correct distance from the platter then the head will read only that track.  However, if the head is too far away it will read multiple tracks, this is called over-scan.  If the heads are too close only part of the track will be read, this is called under-scan.  As an example imagine a paint sprayer and you are spraying a picket fence.  Each slat on the picket fence is a different color.  Now, if you hold the paint gun (head) too far away from the fence (platter) it will spray the neighboring slats (tracks).  On the other hand, if the paint gun is too close you only spray part of the slat.

  With these facts, here is why platter swelling is bad: a platter is not completely flat.  Microscopically there are many flaws in a platter, however, because the heads float on a cushion of air the imperfections do not affect the physics of the read.  If, however, the platter swells then the imperfections become accentuated and the cushion of air can no longer compensate.  In many cases the platter imperfection can become so large that it exceeds the distance of the air cushion and actually touches the head.  This is called a head crash and can scratch the platter, damage the head and wreak all kinds havoc with the drive.

  If your computer is in a room that exceeds eighty degrees then that could cause platter swelling.  If your tower is inside of a desk and the air flow is restricted that will cause platter swelling.  If there is dust in your tower then that restricts airflow and can cause platter swelling. 

  So keep your tower dust free, keep your computer room cool, and keep the tower in an area with good air flow.

  Well, now that I got that off of my technical chest hopefully next time I will cover the the mechanics of finding a stale drive in your RAID five.

  Until next time…
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