Hard Disk

A non-volatile storage device used to store digital data on magnetic surface of rigid plate by using read/write heads.

It is secondary storage device.
Standard size is 3.5 inch for desktops and 2.5 inch for laptops
Modern hard disk uses serial interface like Serial ATA (SATA), Serial Attached SATA (SAS) etc.
Data rate: it is number of bytes transferred per second to the cpu, varies from 5 - 40 MBPS.
Seek time: it is amount of time when cpu request a file and time when cpu gets the first byte of the file, varies from 10 - 20 milliseconds.
Cost of drive is less compare to volatile storage devices like RAM.

Consists of rigid one or more rotating platters coated with magnetic surface, read/write head per platter face to examine the data, spindle on which the platters spins at constant rotational velocity, acutator arm that moves across the platter, IDE connector, power connector, two motors one to spin the disks and other to position the head and a sealed enclosure that protects from dust, condensation etc.
Magnetic surface of each platter is divided into very small magnetic regions called as magnetic domains, which stores the value 0 or 1.
In read/write head, read head element is made up magnetoresistive and write head element is made up of thin-film inductive.
To read a data from the platter, head is moved to a particular magnetic domain where the read head transforms magnetic field into electric current using the property of magnetoresistance.
Similarly to write a data on platter, head is moved to particular magnetic domain where the write head transforms electric current into magnetic field which changes the magnetic polarization to induce either 0 or 1 value.

Internal Hard Disk

Internal Cartridges

To solve problems of internal hard disk, hard disk cartridges came into existence.
Problem such as they can't be removed from the system cabinet easily.
These are easy to remove as like CD from CD drive.
Amount of storage is limited to number of cartridges.
Used as alternative to internal harddisk.
Useful to protect or secure information and also to backup of the pc.
Ranges from 2 GB to 160 GB

Hard Disk Packs

This type of harddisk are used by big organization to store massive amount of data.
Their capacity ranges from Peta Bytes (1 PB = 1024 TB).
Banks and goverment sectors uses to record finicial information.
Consists of serveral platter aligned one above the other.
They resemble stack of phonograph records.
There is space between the disks to allow access arm to move in and out.
Each access arm has two read/write heads.
One reads the disk surface and other reads surface below it.
Only one read/write head is activated at given moment.
Example: Disk pack with 20 disks providing 37 recording surfaces.
Becuase the top and bottom outside surfaces of the pack are not used.

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