We were talking about the first hard disks, way back in 1959...well about 50 years later, we have HDDs in the market almost the size of your palm. Not to forget MMC and RS-MMC of 2 GB! And the 60 GB iPod we carry around...oooo......"tooo much", to quote Vivek Paleria.

Today, we will look into the construction of a HDD.

Construction

Hard disks are rigid platters, composed of a substrate and a magnetic medium.
The substrate - the platter's base material - must be non-magnetic and capable of being machined to a smooth finish.
It is made either of aluminium alloy or a mixture of glass and ceramic.
To allow data storage, both sides of each platter are coated with a magnetic medium - formerly magnetic oxide, but now, almost exclusively, a layer of metal called a thin-film medium.
This stores data in magnetic patterns, with each platter capable of storing a billion or so bits per square inch (bpsi) of platter surface.



Platters vary in size and hard disk drives come in two form factors, 5.25in or 3.5in.
The trend is towards glass technology since this has the better heat resistance properties and allows platters to be made thinner than aluminium ones.
The inside of a hard disk drive must be kept as dust-free as the factory where it was built.
To eliminate internal contamination, air pressure is equalised via special filters and the platters are hermetically sealed in a case with the interior kept in a partial vacuum.
This sealed chamber is often referred to as the head disk assembly (HDA).

If any one of you who had tried to open your HDD, you would have observed your HDD to develop bad sectors and probably stop working.
This is because when u opened the hard case, dust got in...!
A small particle of dirt could cause a head to "crash", touching the disk and scraping off the magnetic coating.


Typically two or three or more platters are stacked on top of each other with a common spindle that turns the whole assembly at several thousand revolutions per minute.
There's a gap between the platters, making room for magnetic read/write head, mounted on the end of an actuator arm.
This is so close to the platters that it's only the rush of air pulled round by the rotation of the platters that keeps the head away from the surface of the disk - it flies a fraction of a millimetre above the disk.
On early hard disk drives this distance was around 0.2mm. In modern-day drives this has been reduced to 0.07mm or less.
On IDE and SCSI drives the disk controller is part of the drive itself.

There's a read/write head for each side of each platter, mounted on arms which can move them towards the central spindle or towards the edge.
The arms are moved by the head actuator, which contains a voice-coil - an electromagnetic coil that can move a magnet very rapidly.
Loudspeaker cones are vibrated using a similar mechanism.

The heads are designed to touch the platters when the disk stops spinning - that is, when the drive is powered off.
During the spin-down period, the airflow diminishes until it stops completely, when the head lands gently on the platter surface - to a dedicated spot called the landing zone (LZ).
The LZ is dedicated to providing a parking spot for the read/write heads, and never contains data.

Enlarged view of a head

...to be contd.