Observe various secondary storage systems- Hard Disk, Flash drives, CD/ DVD drive.

A hard disk drive (HDD), hard disk or hard drive, is something used by computers to store information. Hard disks use magnetic recording (similar to the way recording is done on magnetic tapes) to store information on rotating circular platters. The capacity of a hard drive is usually measured in gigabytes (GB), however hard disc capacity can also be measured in terabytes when the capacity is over 1000 gigabytes. A gigabyte is one thousand megabytes and a megabyte is one million bytes, which means that a gigabyte is one billion bytes. Some hard drives are so large that their capacity is measured in terabytes, (TB) where one terabyte is a thousand gigabytes (1 TB = 1000 GB). Very early Consumer Grade hard drives were measured in megabytes.

Disk Case - The rectangular shaped disk case holds all of the components of a hard disk drive.  The case is secured by screws and should not be opened outside of a dust-free environment.

Disk Platter - The platter shaped like a record with a magnetic surface.  Its job is to store the data contained on the hard disk drive.  There can be one or multiple platters depending on the disk capacity.  A spindle holds the platters together and the motor rotates them at a certain speed.

Spindle - The spindle holds the platters together and the motor rotates the platters at their designated speed, which is measured in RPM.

Actuator - This is designed to move the read/write arm to the correct position on the platter to read the data.

Read/Write Arm - The read/write arm contains many "heads" on the end of the arm which are designed to float above the platter and read data from the platter. 

PLATTER DIVISIONS: 

In order to get maintain the organized storage and retrieval of data the platters are organized into specific structures. These specific structures include tracks, sectors, and clusters.

TRACKS: 

Each platter is broken into thousands of tightly packed concentric circles, known as tracks. These tracks resemble the structure of annual rings of a tree. All the information stored on the hard disk is recorded in tracks. Starting from zero at the outer side of the platter, the number of tracks goes on increasing to the inner side. Each track can hold a large amount of data counting to thousands of bytes.

SECTORS: 

Each track is further broken down into smaller units called sectors. As sector is the basic unit of data storage on a hard disk. A single track typically can have thousands of sectors and each sector can hold more than 512 bytes of data. A few additional bytes are required for control structures and error detection and correction.

CLUSTERS: 

Sectors are often grouped together to form Clusters.

HARD DISK LOGIC BOARD:

Hard disk is made with an intelligent circuit board integrated into the hard disk unit. It is mounted on the bottom of the base casting exposed to the outer side. The read/write heads are linked to the logic board through a flexible ribbon cable.

HSA has precision bearing to make movements nice and smooth. The biggest part of HSA milled from piece of aluminum called the Arm. Heads Gimbal Assembly or HGA attached to the Arm. HGAs and Arms usually produced on different factories. Flexible orange widget called Flexible Printed Circuit or FPC joins HSA and plate with heads contacts.

Let's take closer look on each part of HSA.

The gasket makes connection airtight. The only way for air to go inside HDA is through the breathing hole. On this drive contacts covered with thin layer of gold, for better conductivity.

The black small things at the end of HGAs called Sliders. In many sources you can find that sliders claimed as actual heads but a slider itself is not a head it's a wing which helps read and write elements fly under the platter's surface. Heads flying height on modern HDDs is about 5-10 nanometers. For example: an average human's hair is about 25000 nanometers in diameter. If any particle goes under the slider it could immediately overheat (because of friction) the heads and kill them that's why clean air inside HDA is so important. The actual read and write elements located at the end of the slider and they are so small that can only be seen under a good microscope.

Flash Drive
The flash drive looks small and compact. Yet it is able to store an incredibly huge amount of information. The video below shows the three basic components of a flash drive:

• The board holds all of the internal components and includes the USB connector. The flash drive uses the board as a means of transferring power and data to and from the USB.
• The NAND Flash memory storage chip is the place where all our files are stored.
• The controller chip is the ‘brain’ of the flash drive.
• Retrieves information from the drive and
• Reading/recording information on the NAND flash memory storage chip.

1. USB Connector: USB (Universal Serial Bus) acts as an interface between the NAND flash memory chip and the computer the flash drive is plugged into.
2. USB mass storage controller (or the controller chip): explanation above
3. Test points: These are electrical pins that serve only as electrical contact points. They are primarily used to electrically stimulate and exercise the flash drive during the assembly process – serving as a means of identifying any defects on the board. Think of it as a cost-effective way of Quality Control.
4. NAND flash memory chip: explanation above
5. Crystal Oscillator: It is a piece of quartz crystal designed to vibrate at a very particular frequency. It is used as a digital “clock”.
6. LED indicator light: Used to indicate if the flash drive is working and functioning
7. Write-protect switch: An optional component used to safeguard the information saved on the flash drive.
8. Space to put a second NAND flash memory chip: An additional slot for manufacturers to put another memory chip which can increase the storage capacity

CD/DVD Drive:

1. Inside your CD player, there is a miniature laser beam (called a semiconductor diode laser) and a small photoelectric cell (an electronic light detector). When you press play, an electric motor (not shown in this diagram) makes the disc rotate at high speed (up to 500rpm). The laser beam switches on and scans along a track, with the photocell, from the center of the CD to the outside (in the opposite way to an LP record). The motor slows the disc down gradually as the laser/photocell scans from the center to the outside of the disc (as the track number increases, in other words). Otherwise, as the distance from the center increased, the actual surface of the disk would be moving faster and faster past the laser and photocell, so there would be more and more information to be read in the same amount of time.
2. The laser (red) flashes up onto the shiny (under) side of the CD, bouncing off the pattern of pits (bumps) and lands (flat areas) on the disc. The lands reflect the laser light straight back, while the pits scatter the light.
3. Every time the light reflects back, the photocell (blue) detects it, realizes it's seen a land, and sends a burst of electric current to an electronic circuit (green) that generates the number one. When the light fails to reflect back, the photocell realizes there is no land there and doesn't register anything, so the electronic circuit generates the number zero. Thus the scanning laser and electronic circuit gradually recreates the pattern of zeros and ones (binary digits) that were originally stored on the disc in the factory. Another electronic circuit in the CD player (called a digital to analog converter or DAC) decodes these binary numbers and converts them back into a changing pattern of electric currents.
   
4. A loudspeaker transforms the electric currents into sounds you can hear (by changing their electrical energy into sound energy).

CDs are made from an original "master" disc. The master is "burned" with a laser beam that etches bumps (called pits) into its surface. A bump represents the number zero, so every time the laser burns a bump into the disc, a zero is stored there. The lack of a bump (which is a flat, unburned area on the disc, called a land) represents the number one. Thus, the laser can store all the information sampled from the original track of music by burning some areas (to represent zeros) and leaving other areas unburned (to represent ones). Although you can't see it, the disc holds this information in a tight, continuous spiral of about 3-5 billion pits. If you could unwrap the spiral and lay it in a straight line, it would stretch for about 6 km (roughly 3.5 miles)! Each pit occupies an area about two millionths of a millionth of a square meter.