HARD DISK

HARD DISK DRIVES: -

Today Hard disk drives are the most common storage device used with the computer system. The hard disk drive is also known as hard disk, hard drive,fixed disk drive or Winchester disk drive. The hard disk drive is used to store data and programs permanently inside the computer. The information stored in the hard disk drive does not get erased when the power supply to the computer is switched off.
This device is known as “Hard Disk Drive” because compared to the other common magnetic storage device floppy disk, which used flexible plastic like carrier for the magnetic recording medium, the hand disk drive uses rigged, hard, aluminum or glass as the base for recording medium.

COMPONENTS OF HARD DISK DRIVE: -

There are many different size and capacity hard disk drive available in market, but the basic component inside them are almost same. A typical hard disk drive contains the following components: -

1. PLATTER
2. READ/WRITER HEAD
3. HEAD ARM/HEAD SLIDER
4. HEAD ACTUATOR MECHANISM
5. SPINDLE MOTOR
6. LOGIC BOARD
7. AIR FILTER
8. BEZEL
9. CABLE & CONNECTORS

1.DISK PLATTER: -

1. Platters are relatively heavy-duty disks of aluminum, glass, or ceramic composite material
2. Platters are coated on both side with a layer of magnetic material (called media) and covered with a protective layer.
3. Finished and polished platters are they stacked and coupled to the spindle motor.
4. Some drive might use only one platter.
5. The r/w head assembly is fitted in between each disk platter
6. There is usually one head per platter side, so a drive with two platters should have four heads.
7. During drive operation, the platter spin at a speed from 5200 RPM to 10,000 RPM

There were many different platters sizes in use, but some of the common platters sizes are given below

2. THIN FILM MEDIA: -

The limitations of the iron oxide coating has made the hard drive Manufactures to look for some alternative, and the thin film media coating in the result of this the thin film media is created on the platter surface using two different processes they are,

1. Plating process
2. Sputtering process.

1. PLATING PROCESS: -

In the platting process, as the name suggest the media is produced
Using electroplating process. In this process the platter substrate is immersed in different chemicals to coal the platter surface with a very uniform 2 to 3 micro inch thick cobalt alloy coating.

1. SPUTTERING PROCESS: -

The sputtering process provides better thin film coating compared
to plating process. This process provides thinnest, hardest and finest media surface. In this process the platter substrate is first coated with a layer of nickel phosphorus and them on this surface cobalt alloy material is deposited using a sputtering process.

2 ) READ/WRITE HEAD:-

1. Read/Write heads form the interface between a drive’selectronic circuitry and magnetic media.
2. During writing, ahead translates electronic signals into magnetic-flux transitions that saturate points on the media, where those transitions occur.
3. A read operation works roughly in reverse. Flux transitions along the disk induee electrical signaleds in the head, which are amplified, filtered, and translated into corresponding logic signals.
4. Currently used r/w head is very flat, sensitive, small and durable, but even thin film heads use an air gap and 8 to 34 turns of copper wire.
5. The heads are attached to long metal arms that are moved b7 the head actuator motor.
6. The entire subassembly is sealed in the platter compartment, and in generally inaccessible, unless it is opened in a clean room environment.

IFFERENT TYPE OF HEAD: -

1. Ferrite head
2. Metal-in gap head
3. Thin film head
4. Magnets-resistive head

FERRITE HEAD: -

This was the original hard disk drive head made by the IBM for their Winchester disk drives. This head is made of iron oxide core wrapped with elute magnetic coils. To write any information sing this head, the coil is energized, which produces magnetic shield on the disk surface. To read the information stored on the disk surface this head passed over the disk surface and the induced current generated in the coil in used to read the data

METAL-IN-GAP HEAD: -

After the ferrite head technology next popular technology was metal-in-gap head. As the name suggests in this type of head, a metal allow is placed in the recording gap, on the trailing edge of the head. This metal is placed using the calcium depositing process called sputtering.
Metal-in-gap head could write to hang density thin film media used in high capacity drives. But as the drive capacity increased there the thin-film head replaced heads.

THIN-FILM HEAD:-

Thin film head are very small and lightweight heads which can be used as close as 2U inch or less to the disk surface. These head are produced using a photolithographic process similar to the process used to make semiconductor chips.,

MAGNETO-RESISTIVE HEAD: -

This head is new design, invented by the IBM. This design is considered superior to all other current head designs contently most of the 3 ½ inch drives with 1 GB or more capacity use this head design, as the storage density of the hard disk drives increase further, slowly all the dives will use the magneto-revisited head

3) HEAD ARM/ HEAD SLIDER:-

The arm on which the R/W head of the disk drive is located is called the “head slider”. Their sliders are made in a catamaran sailboat shape.
A common slider size in today 3-½ inch hard disk drive is 0.08*0.017 inch. Slider of this size is called a “Nano slider” because of its small size.

4) SPINDLE MOTOR:-

1. A spindle motor in typically a DC motor.
2. The motor is responsible for spinning the platter
3. An index sensor provides feed back pulse signal that detects the spindle as it rotates
4. Media is passed under the R/W heads by spinning the platters at a high rate of speed ( at least 3600rmp. To as high as 10000rmp)
5. The driver’s control electronics uses the index signal to regulate spindle speed as precisely as possible
6. On e of the major factors that contribute to hard drive performances is the speed at which media passes under the R/W heads.
7. The spindle motor and index sensor are also scaled in the platter compartment

5) LOGIC BOARD: -

Other than the read/write head, spindle motor, head movement assembly, the hard disk also contains a logic board t control all these different parts of the hard disk drive. This logical board is also used to interface the hard disk drive with the computer. A hard disk drive may contain one, two or even their logic boards at the top at the back and at the bottom to the drive.

6) AIR FILTER: -

Most of the hard disk drive when used in very attitude, such as mountaintops, or inside airplane, where air pressure is very low, will not work. For this type of application, one would require special hermetically sealed airtight hard disk drives, which will have air inside the hard disk drive, buy is completely sealed from the outside most of the hard disk drive will have two-air filter one is called reticulating air filter and second is called breather filter,

7) BEZEL/FRONT FACEPLATE:-

Bezel is the front faceplate provided on most of the hard disk drives. Now days as most of the hard disk are connected internally and is hidden behind the pc systems cover the faceplate or the bezel is not needed. most of the hard disk drive today come without a bezel.

8) CABLE & CONNECTORS:-

Cables and connectors are used to connect the hard disk drive to the main computer system. Almost all the hard disk drive contains connector for 1) Data/Control (40 pin)interface connector, 2) Power connector.

9) HEAD ACTUATO MECHANISM:-

The read/Write head of the hard disk drive is moved on the platter surface using different mechanism, this is referred as head actuator mechanism.

Two different head actuator mechanism in use are

1. Stepper motor actuator
2. Voice coil Actuator.

This actuator mechanism work on completely different principle. The head actuator is one of the most important parts of the hard drive, because different characteristic of the hard disk such as its r/w speed, accuracy, reliability etc. very much depend on the head actuator mechanism used into that drive.

STORAGE CAPACITY: -

A hard disk capacity can be expressed in the following four different ways: -

1. Unformatted storage capacity in millions of bytes.
2. Formatted storage capacity in millions of bytes.
3. Unformatted storage capacity in megabytes.
4. Formatted storage capacity in megabytes.

HARD DISK DRIVE RELATED TERMS: -

Disk Head: -

Hard disk has an electromagnetic read/write head for each side of each platter. So a drive with two platters should have three or four heads.

Sectors: -

Each surface is divided in pie shaped areas. Both hard and floppy devices store 512 bytes in sector that is 0.5 k data as 1kb=1024 bytes

Track: -

Each side of the hard disk drives platters surface is divided into concentric circles called track. There tracks are not visible marks on the disk surface instead these are magnetic information written during the formatting of hard disk drive. The outermost track number and the next track is 1,next track 2 and so on, the innermost track will have the highest number. Number of track on hard disk may range from 300 to 3000.

Cylinders: -

All tracks under read/write had at one time (two tracks for diskettes)
The number of tracks per surface is identical to the number of cylinders, so most manufactures do not report the number of tracks they report the number of cylinders.
Where data is stored on the hard disk drive, it is stored in cylinder by cylinder i.e. first all the tracks of same cylinder is written, once a cylinder becomes full the read/write had moves to the next cylinder and writer balance data to the next cylinder.

COMPARISON OF DRIVE PARAMETERS VS. CAPACITY.

ZONED RECORDING: -

1. In the early days of hard drives, every track had the same number of sectors (i.e. 64 or through 63)
2. Designers realized that in these drives the data was recorded more densely on the inner tracks, where the circumference is lower, and less density on the outer tracks, where the circumference is higher.
3. A feature known as zoned recording was added to the drive which allows a variable number of tracks
4. The total number of tracks is divided up into a number of “zones”
5. All of the tracks with in zone use the same number of sectors, but inner zones use fewer sectors and outer zones use more sectors.
6. Current hard drives can run from 195 to 312 physical sectors per track.

Drive Formatting: -

You can imagine a disk drive as being a big file cabinet, when the drive is first installed the file cabinet is completely empty there are no dividers. Or folder or labels of any kind to organize information’s to make the drive useful, it must be formatted and partitioned
There are basic all three steps to the format process.

1. A low-level format
2. Partitioning
3. A high-level format

1. Low-Level Formatting: -

Sectors header and trailer information is written along with dummy
data. Inner-sector and inter-track gaps are aloes created. Because this information’s only written once, age and wear can allow sector info4mation to eventually fail, when this happens, the failed sector’s are unreadable. This problem is further compounded by the fact that low level formatting is hardware. Specific and most current drive makers, low level format their drives at the factory.
If you attempt to invoke low-level IDE/EIDE formatting with Dos debug
Sequence or software utility, one of four things might happen.

1. The drive will ignore the low level formatter entirely
2. The drive will accept the formatter, but only erase areas containing data
3. The drive will accept the formatter and perform a correct low-level format. This is highly unlikely.

2) Partitioning: -

Partitioning is an operating system specific process. After low level formatting is complete, the drive must be partitioned before can os file system or boot information is written to the drive.
Several file systems are in service today but Dos Windows 3.1y and window 95 continues to use the FAT system. The main criticism of the FAT is that sectors are grouped and aligned as clusters. One of the newly created partitions will be assigned as the boot partition, and master boot sector (MBS) containing a special boot program and partition table will be written to the first sector.
FDISK is the DOS utility used for drive partitioning

3) High-Level (DOS) Formatting: -
After partitions, an operation system cannot store files on a drive. A series data structures must be written to the drive. A volume Boot sector (VBS), two copies of the FAT and a root directory are written to each logical partition. High level formatting also checks and locks out bad sectors so that they will not be used during normal operation. Format is the Dos utility used for high level formatting

Cluster: -

When Dos writes some inforamtion onto the hard disk, it does not allocate the space sector wise instead it used a new unit of storage called clusters. Cluster are the min. space allocated by Dos when storing any information on the disk.

File Systems: -

When you purchase an IDE/EIDE or SCSI hard drive today, it is already low-level for matted, the cylinder track and sector information’s already written on the drive
The various file allocation table are: -
1) FAT 16
2) FAT32
3) NTFS