front and back of system unit

Front;

1.Restart button

2.Poert button

3.OPtical media

4.Status indicator

5.Auxiliary port

6.USB port

Back;

1.Power Supply unit
2.Ps/2 ports
3.USB port
4.Parallel port
5.Serial port
6.AGP port
7.Lan port
8.Expansion slot

The Hard Disks

A Hard disk drive (HDD), commonly referred to as a hard drive, hard disk, or fixed disk drive, is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.
HDDs (introduced in 1956 as data storage for an IBM accounting computer were originally developed for use with general purpose computers. In the 21st century, applications for HDDs have expanded to include digital video recorders, digital audio players, personal digit assistants, digital cameras and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia.The need for large-scale, reliable storage, independent of a particular device, led to the introduction of embedded systems such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.


Platters
The actual storage media in the different types of disks. In a hard drive the platter haa a core of glass or aluminium covered with a thin layer of Ferric oxide or a Cobalt alloy (Co-Ni, Co-Cr, Co-Ni-W). This layer is protected by a layer of a very hard material (overcoat), and a thin layer of lubricant. A CD is a plastic disc in which the data is impressed. It has a metallic, reflecting backside.



Spindle
The platters are mounted by cutting a hole in the center and stacking.


Slider
The special electromagnetic read/write devices.

Actuator Arm
The hard drive's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback. These are either complete concentric circles (in the case of dedicated servo technology), or segments interspersed with real data (in the case of embedded servo technology). The servo feedback optimizes the signal to noise ratio of the GMR sensors by adjusting the voice-coil of the actuated arm. The spinning of the disk also uses a servo motor. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.

Actuator
is a mechanical device for moving or controlling a mechanism or system.