Hard Disk Drive - An Inner look

HARD DISK DRIVE
Hard disk is a magnetic disk storage device that stores digital data on rotating magnetic surfaces. These are permanently mounted or "fixed" in the system and are not removable like floppy disks. The computer’s operating system and other software are installed on the hard disk. Since, the disk and the drive are usually contained in the same unit, hard disk is also known as "hard drive". Hard disks store more data and provide quick access to large quantities of data than floppy disks. Hard disks are needed to be formatted before they can store data.

Function:
The hard drive is a permanent storage space for data and the programs that are used to create the data. Inside the hard disk, both sides of an individual platter are covered with a special magnetic material located. These platters are fixed to the spindle and they spin at thousands of Revolution per minute (RPM)

The overall performance of the hard drive is dependant on the Revolutions Per Minute, which means the total number of revolutions made by a platter in 60 seconds. The speed at which the data can be read from the platters is directly proportional to the RPM. The values of RPM range from 5,400RPM to 12,000RPM and more.

Description:
Hard disk drive, a disk controller, jumpers, and a cable comprise a Hard Disk Drive system. Several computers have controller as a separate expansion board, which interfaces the system through an expansion slot. Direct cabling to the power outlet provides the power supply. Several hard drive interfaces are IDE, SCSI, IEEE 1394, USB etc.

Hard drives are available with several different storage capacities. The storage capacity of a hard drive is measured in megabytes (MB) and gigabytes (GB).The common storage capacities these days range from 40 GB to 120 GB and more. An ideal drive would be physically small, have fast seek times, spin fast, large buffers and a long warranty.

The drives that are connected externally by a cable to a special port are known as "External drives". The most common external drives are

HARD DRIVE PHYSICAL COMPONENTS:

PLATTERS:
Platter is a circular, metal disk that is mounted inside a hard disk drive. Several platters are mounted on a fixed spindle motor to create more data storage surfaces in a smaller area. The platter has a core made up of aluminium or glass substrate, covered with a thin layer of Ferric oxide or cobalt alloy. On both sides of the substrate material, a thin coating is deposited by a special manufacturing technique. This, thin coating where actual data is stored is the media layer. When the magnetic media is applied to the surface of the substrate material, a thin lubricating layer is applied to protect the material. This complex three layered media is discussed in detail as follows:

THE SUBSTRATE MATERIAL:
The bulk material of which platters are made up, forms the base on which media layer is deposited. The substrate has no specific function but to support the media layer. The most commonly used material for making this physical layer is an Aluminium alloy. This alloy is rigid, lightweight, stable, inexpensive, easy to work with and is readily available. Earlier, since the gap between the heads and the platter was relatively high, the platter surface being smooth and flat was less of an issue. However, as technology advances, the gap between heads and platters is decreasing and the speed that the platters spin at is increasing. For this reason demand for alternatives on the platter material are increasing. Glass platters are replacing aluminium platters because they provide improved rigidity, better quality, thinner platters, and thermal stability.

MEDIA LAYER:
The substrate material forms the base upon which actual recording media is deposited. The media layer is a thin coating of magnetic material applied to the surface of the platters and where the actual data is stored. Its thickness is only a few millionths of an inch.

Special techniques are employed for the deposition of magnetic material on the substrate material. A thin coating is deposited on both sides of the substrate, mostly by vacuum deposition process called magnetron sputtering. Another such method is electroplating, using a process similar to that used in electroplating jewelry.

PROTECTIVE LAYER:
On the top of the magnetic media, is applied a super-thin, protective, lubricating layer. This layer is called the protective layer because it protects the disk from damage caused by accidental contact from the heads, "head crash" or other foreign material from entering the drive

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.

READ/WRITE HEADS:
The heads are an interface between the magnetic media where the data is stored and electronic components in the hard disk. The heads convert the information, which is in the form of bits to magnetic pulses when it is to be stored on the platter and reverses the process while reading.

The heads are the most sophisticated part of the hard disk. Each platter has two read/write heads, one mounted on the top and the other one at the bottom. These heads are mounted on head sliders, which are suspended at the ends of head arms. The head arms are all fused into a singular structure called actuator, which is responsible for their movement.

THE SPINDLE MOTOR:
Spindle motor plays an important role in hard drive operation by turning the hard disk platters. A spindle motor must provide stable, reliable, and consistent turning power for many hours of continuous use. Many hard drive failures occur due to spindle motor not functioning properly

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.

DRIVE BAY:
The entire hard disk is mounted in an enclosure designed to protect it from the outside air. It is necessary to keep the internal environment of the hard disk free of dust and other contaminants. These contaminants may get accumulated in the gap between the read/write heads and the platters, which usually leads to head crashes.

The bottom of the disk is also called base casting. The drive mechanics are placed in the base casting and a cover, usually made up of aluminium is placed on top to enclose heads and platters. The entire contents placed on the base and cover chamber are collectively known as the head-disk assembly. Once this assembly is opened, it would instantly contaminate the contents and eventually ruin the drive.
On the bottom of the base casting is present the logic board, which is separated from the base casting using a cushioning material.

TYPES OF HARD DRIVE CONNECTIONS/INTERFACES:

• SCSI
• IDE/EIDE/ATA/PATA
• SATA
• USB
• FireWire/IEEE 1394
• Fiber

SCSI:
Stands for small computer system interface and is a parallel interface standard used for attaching peripheral devices to computers at the same time, such as hard drives, printers, scanners, CD-ROM/RW drives, DVD drives and tape drives. SCSI connector provides for faster data transmission rates up to 320 Megabytes per second (320 MBps). SCSI provides higher data transfer rates than ATA.

SCSI SPECIFICATIONS:

There are three basic specifications:

SCSI-1:
SCSI-1 features an 8-bit parallel bus running at 3.5 MB/s in an asynchronous mode and 5 MB/s in a synchronous mode. SCSI-1 supports 8 devices at the maximum with the maximum cable length of 6 meters.

SCSI-2:
It features a 16-bit bus width doubling the maximum transfer rate to 10 MB/s. SCSI-2 supports up to 16 devices with the maximum cable length of 3 meters.

SCSI-3:
SCSI-3 uses a 16-bit bus and the maximum transfer rate to 320 MBps. It is also called Ultra Wide SCSI.Other specifications include: Ultra2 SCSI, Wide Ultra2 SCSI, Wide SCSI, Fast SCSI, Fast Wide SCSI, Ultra SCSI.

IDE/EIDE/ATA/PATA:
IDE (Integrated Drive Electronics); EIDE (Enhanced Integrated Drive Electronics); ATA (Advanced Technology Attachment); PATA (Parallel advanced technology attachment). It is an interface for mass storage devices, in which the controller is integrated into the hard disk, CD-ROM drive, and the floppy drive.The IDE interface uses advanced technology attachment interface for cable lengths up to 2 feet. A single IDE ATA channel supports up to two drives, master and the slave. At a time, IDE can only access one drive per channel.

IDE is less expensive than SCSI but offers less performance level. IDE interface could support drives up to 540 MB

SATA:
Serial ATA is an evolution of the parallel ATA interface. A single cable with atleast four wires creates a point-to-point connection between devices, which defines that serial ATA is a serial link. Serial ATA support data transfer rates starting from 150 MBps. Serial ATA cables can extend up to one meter. This interface supports all ATA and ATAPI devices. Serial ATA has thinner cables, hence, allow for smaller chassis designs. These cables can extend up to one meter.

Serial ATA is not backward compatible i.e. it will not work with the same connectors that IDE, SCSI or any other interface.

USB: Stands for Universal Serial Bus is an external serial bus standard to interface devices. USB is an external bus that supports three data transfer rate:

• A low speed rate of 1.5 Mbps, mostly used for human interface devices, such as mice, keyboards, and joysticks.
• A full speed rate of 12 Mbps supported by all USB Hubs.
• A high speed rate of 480 Mbps. The high speed devices are commonly referred to as USB 2.0.

A single USB port is capable of connecting up to 127 peripheral devices, such as printers, scanners, digital cameras, mice, keyboards, joysticks, modems, speakers, telephone, video phones etc.

Connecting a USB device to the computer requires the plugging of the USB device cable connector to the USB connector at the back of the computer. A typical USB socket at the back of the computer is rectangular. A USB standard uses two different connectors, "A" and "B". "A" connectors head upstream toward the computer and B connectors head downstream and connect to individual devices.

The main features of USB include:

+ Ease of Use.
+ The computer acts as a host.
+ Up to 127 devices can be connected to the host with maximum cable length of 5 meters for individual USB and 30 meters for USB hubs.
+ USB devices are hot swappable, i.e. they can be plugged and unplugged at any time.
+ USB supports Plug-and-Play installation.
+ High data transfer rate of 480 megabits per second with USB 2.0.

FireWire/ IEEE 1394:

The IEEE 1394 standard for High Performance Serial Bus, also called FireWire, is a way of transferring information between digital devices, mainly audio and video equipment.
FireWire is a very fast external bus that supports data transfer rates up to 400 Mbps via IEEE 1394a (FireWire 400), which is about 3 times faster than USB. A single IEEE 1394 port is capable of connecting up to 63 external devices with cable length up to about 4.5 m. IEEE 1394b (FireWire 800) is capable of transferring data up to 800 Mbps with the maximum cable length up to 100 meters. The 1394b standard is faster than 1394a and is backward-compatible. FireWire is plug-and-play, which means if a new FireWire device is connected to the computer, the operating system detects it automatically, or if any new device has been installed, the computer activates it. FireWire devices are hot pluggable, i.e. it allows plugging and unplugging devices at any time, even when the power is on.

The main features of FireWire include:

• Supports high data rate
• Plug and play
• Hot-pluggable
• Inexpensive
• Easy to use
• Ability to connect lot of devices on the bus
• Isochronous data transfer

In Isochronous mode, multiple, time-critical, multimedia data streams are delivered at a guaranteed rate.

FC-AL (Fiber Channel-Arbitrated Loop):
FC-AL is designed for high-bandwidth high end systems and is compatible with mass storage devices and other peripheral devices that require very high bandwidth. It supports serial mode of data transfer.

This interface uses optical fiber cable to connect the devices to produce a maximum data transfer rate of more than 100 MB/second. FC-AL loop links as many as 127 devices as far as 10 kilometers apart. FC-AL can be connected to two ports to double the data transfer rates. This interface is more expensive as compared to SCSI interface.

All devices in an arbitrated loop are similar to token ring networking. When one device stops functioning, the entire loop gets interrupted. To overcome this failure, channel hubs are present to connect multiple devices together.

The main features of FC-AL include:

• Serial data transfer for distances greater than 10 Km
• High data transfer rate
• Supports for a large number of devices
• Reliable for data transfer
• Compatibility with various already existing protocols
• Frames (data packets)

About the author:
Park Oskar a freelancer writer for http://www.stellarinfo.com, a leading provider of file recovery software & service provider. Download recovery software Demo to recover lost, deleted or corrupted files.