Hard Disk Platters
Every hard disk contains one or more flat disks that are used to actually hold the data in the drive. These
disks are called platters (sometimes also "disks" or "discs"). They are composed of two main
substances: a substrate material that forms the bulk of the platter and gives it structure and rigidity,
and a magnetic media coating which actually holds the magnetic impulses that represent the data. Hard disks
get their name from the rigidity of the platters used, as compared to floppy disks and other media which use flexible
|Typical track is shown in yellow
|Typical sector is shown in blue
Data is stored on the surface of a platter in
sectors and tracks. Tracks are concentric circles, and sectors are pie-shaped wedges on a track.
A sector contains a fixed number of bytes -- for example, 256 or 512. Either at the drive or the operating system
level, sectors are often grouped together into clusters.
The process of low-level formatting a drive establishes the tracks and sectors on the platter. The starting and
ending points of each sector are written onto the platter. This process prepares the drive to hold blocks of bytes.
High-level formatting then writes the file-storage structures, like the file-allocation table, into the sectors.
This process prepares the drive to hold files.
Platter Substrate Materials
The magnetic patterns that comprise your data are recorded in a very thin media layer on the surfaces of
the hard disk's platters; the bulk of the material of the platter is called the substrate and does nothing but
support the media layer. To be suitable, a substrate material must be rigid, easy to work with, lightweight, stable,
magnetically inert, inexpensive and readily available. The most commonly used material for making platters has
traditionally been an aluminum alloy, which meets all of these criteria.
Hard disk platters viewed with a scanning electron
The image on the left is of the surface of an aluminum alloy platter.
The image on the right is a glass platter.
The scale is in microns.
Hard disks can have one platter, or more, depending on the design. Standard consumer hard disks usually have between
one and five platters in them. In every drive, all the platters are physically connected together on a common central
spindle to form a single assembly that spins as one unit, driven by the spindle motor. The platters are kept apart
using spacer rings that fit over the spindle. The entire assembly is secured from the top using a cap or cover
and several screws.
Each platter has two surfaces that are capable of holding data; each surface has a read/write head. Normally both
surfaces of each platter are used, but that is not always the case. Some older drives that use dedicated servo
positioning reserve one surface for holding servo information. Newer drives don't need to spend a surface on servo
information, but sometimes leave a surface unused for marketing reasons--to create a drive of a particular capacity
in a family of drives. With modern drives packing huge amounts of data on a single platter, using only one surface
of a platter allows for increased "granularity". For example, Hard disk of 40GB family have an impressive
20 GB per platter data capacity but a 30 version of this drive uses three surfaces (on two platters) for that drive.
From an engineering standpoint there are several factors that are related to the number of platters used in the
disk. Drives with many platters are more difficult to engineer due to the increased mass of the spindle unit, the
need to perfectly align all the drives, and the greater difficulty in keeping noise and vibration under control.
More platters also means more mass, and therefore slower response to commands to start or stop the drive.
Data Flow on the Disk
This is how hard disk executes data, when a command is made to store or retrieve some data on and from a disk,
the following chain of events occurs:
The data flows into a cache where it is encoded using special mathematically derived formulae, ensuring that any
subsequent errors caused by noise can be detected and corrected.
Free sectors on the disk are selected and the actuator moves the heads over those sectors just prior to writing.
(The time it takes the actuator to move to the selected data track is called the "seek" time.)
Once over the data track, the heads must not write the data until the selected free sectors on that track pass
beneath the head. This time is related to the rotation speed of the disk: the faster the speed, the shorter this
When it's time to write, a pattern of electrical pulses representing the data pass through a coil in the writing
element of the recording head, producing a related pattern of magnetic fields at a gap in the head nearest the
disk. These magnetic fields alter the magnetic orientations of bit regions on the disk itself, so the bits now
represent the data.
When a command is made to read some data on a disk, a similar process occurs in reverse. After consulting the table
of stored data locations in the drive's electronics, the actuator moves the head over the track where the chosen
data is located. When the correct sectors pass beneath the head, the magnetic fields from the bits induce resistivity
changes in the sensitive MR or GMR materials located in the reading elements within the head. These elements are
connected to electronic circuits, and the current flowing through those circuits change with the resistivity changes.
The current variations are then detected and decoded to reveal the data that had been stored on the disk.
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