Practical No 5

Aim: Study of Hard Disk Drive(HDD). Introduction:

A hard 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. A hard drive is a mass storage device found in all PCs (with some exclusions) that is used to store permanent data such as the operating system, programs and user files. The data on hard drives can be erased and/or overwritten, the hard drive is classed as a non-volatile storage device which means it doesn't require a constant power supply in order to retain the information stored on it (unlike RAM). The hard drive works in parallel with the memory (RAM) and the motherboard to read and write data through the system. Operating systems and applications including games requirements for disk space has increased vastly in recent times. An example of this is a typical Windows XP install uses approx 1.5GB of space, a typical PC DVD Game will need just under 5GB.

History: HDDs (introduced in 1956 as data storage for an IBM accounting computer) were originally developed for use with general purpose computers. During the 1990s, 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. In the 21st century, HDD usage expanded into consumer applications such as camcorders, cell phones (e.g. the Nokia N91), digital audio players, digital video players (e.g. the iPod Classic), digital video recorders, personal digital assistants and video game consoles. NOTE: The HDD is one PC component that even the most intrepid do-it yourself should not attempt to open and repair. Factory technicians working in dust-free, environmentally pure clean rooms are the only people who should open the drive casing.

Technology: HDDs record data by magnetizing ferromagnetic material directionally, to represent either a 0 or a 1 binary digit. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called platters, onto which the data are recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic material. Older disks used iron (III) oxide as the magnetic material, but current disks use a cobalt-based alloy. . The platters are spun at very high speeds. Information is written to a platter as it rotates past devices called read-and-write heads that operate very close (tens of nanometers in new drives) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm.

An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or in some older designs a stepper motor. Older drives read the data on the platter by sensing the rate of change of the magnetism in the head; these heads had small coils, and worked (in principle) much like magnetic-tape playback heads, although not in contact with the recording surface. As data density increased, read heads using magneto-resistance (MR) came into use; the electrical resistance of the head changed according to the strength of the magnetism from the platter. Later development made use of spintronics, in these heads, the magneto-resistive effect was much greater that in earlier types, and was dubbed "giant" magneto-resistance (GMR). This refers to the degree of effect, not the physical size, of the head — the heads themselves are extremely tiny, and are too small to be seen without a microscope. GMR read heads are now commonplace. HD heads are kept from contacting the platter surface by the air that is extremely close to the platter; that air moves at, or close to, the platter speed.[citation needed] The record and playback head are mounted on a block called a slider, and the surface next to the platter is shaped to keep it just barely out of contact. It's a type of air bearing.

Hard Disk Drive

The magnetic surface of each platter is conceptually divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. In today's HDDs, each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localized magnetic field nearby. The write head magnetizes a region by generating a strong local magnetic field. Early HDDs used an electromagnet both to generate this field and to read the data by using electromagnetic induction. Later versions of inductive heads included metal in Gap (MIG) heads and thin film heads. In today's heads, the read and write elements are separate, but in close proximity, on the head portion of an actuator arm. The read element is typically magneto-resistive while the write element is typically thin-film inductive.[6] In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom-thick layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.[7] Another technology used to overcome thermal effects to allow greater recording densities is perpendicular

recording, first shipped in 2005,[8] as of 2007 the technology was used in many HDDs.[9][10][11] Modern drives also make extensive use of Error Correcting Codes (ECCs), particularly Reed–Solomon error correction. These techniques store extra bits for each block of data that are determined by mathematical formulas. The extra bits allow many errors to be fixed. While these extra bits take up space on the hard drive, they allow higher recording densities to be employed, resulting in much larger storage capacity for user data. Cylinders: A cylinder comprises the same track number but spans all such tracks across each platter surface that is able to store data (without regard to whether or not the track is "bad"). Thus, it is a three-dimensional object. Any track that comprises the same cylinder can be written to and read from while the actuator assembly remains stationary. One way drive makers have been able to increase drive speed is by increasing the number of platters that can be read at a given time. Sectors: Each usable side of a platter can also be thought of as a collection of slices called sectors.

Cylinders, Sectors, Tracks, and Head of HDD Tracks: The tracks are the thin concentric circular strips on a floppy medium or platter surface which actually contain the magnetic regions of data written to a disk drive. They form a circle and are (therefore) two-dimensional. At least one head is required to read a single track. Heads: Data is written to and read from the surface of a platter by a device called a head. Naturally, a platter has 2 sides and thus 2 surfaces on which data could be manipulated; usually there are 2 heads per platter--one on each side, but not always.

(Sometimes the term side is substituted for head, since platters might be separated from their head assemblies; as is definitely the case with the removable media of a floppy drive.) Block: The intersection of a track and a sector is called a block. Thus, blocks are delimited by specifying a certain cylinder, head and sector. These blocks are the smallest geometrical breakdown of a disk, and represent the smallest amount of data which can be transferred to or from a disk (usually 512 bytes). Platters: Every hard disk contains one or more flat disks that are used to actually hold the data in the drive. These disks are called platters (sometimes also "disks" or "discs"). They are composed of two main substances: a substrate material that forms the bulk of the platter and gives it structure and rigidity, and a magnetic media coating which actually holds the magnetic impulses that represent the data. Hard disks get their name from the rigidity of the platters used, as compared to floppy disks and other media which use flexible "platters" (actually, they aren't usually even called platters when the material is flexible.) The platters are "where the action is"--this is where the data itself is recorded. For this reason the quality of the platters and particularly, their media coating, is critical. The surfaces of each platter are precision machined and treated to remove any imperfections, and the hard disk itself is assembled in a clean room to reduce the chances of any dirt or contamination getting onto the platters. Platter size: The size of the platters in the hard disk is the primary determinant of its overall physical dimensions, also generally called the drive's form factor; most drives are produced in one of the various standard hard disk form factors. Disks are sometimes referred to by a size specification; for example, someone will talk about having a "3.5-inch hard disk". When this terminology is used it usually refers to the disk's form factor, and normally, the form factor is named based on the platter size. The platter size of the disk is usually the same for all drives of a given form factor, though not always, especially with the newest drives, as we will see below. Every platter in any specific hard disk has the same diameter. The first PCs used hard disks that had a nominal size of 5.25". Today, by far the most common hard disk platter size in the PC world is 3.5". Actually, the platters of a 5.25" drive are 5.12" in diameter, and those of a 3.5" drive are 3.74".

Types of Heads: There are basic four types of Heads: (1) Ferrite Heads: The oldest head design is also the simplest conceptually. A ferrite head is a U-shaped iron core wrapped with electrical windings to create the read/write head--almost a classical electromagnet, but very small. (The name "ferrite" comes from the iron of the core.) The result of this design is much like a child's U-shaped magnet, with each end representing one of the poles, north and south. When writing, the current in the coil creates a polarized magnetic field in the gap between the poles of the core, which magnetizes the surface of the platter where the head is located. When the direction of the current is reversed, the opposite polarity magnetic field is created. For reading, the process is reversed: the head is passed over the magnetic

fields and a current of one direction or another is induced in the windings, depending on the polarity of the magnetic field. (2) Metal-in-Gap Heads: An evolutionary improvement to the standard ferrite head design was the invention of Metal-In-Gap heads. These heads are essentially of the same design as ferrite core heads, but add a special metallic alloy in the head. This change greatly increases its magnetization capabilities, allowing MIG heads to be used with higher density media, increasing capacity. While an improvement over ferrite designs, MIG heads themselves have been supplanted by thin film heads and magneto-resistive technologies. They are usually found in PC hard disks of about 50 MB to 100 MB. Note: The word "gap" in the name of this technology refers to the gap between the poles of the magnet used in the core of the read/write head, not the gap between the head and the platter. (3) Thin-Film Heads: Thin Film (TF) heads--also called thin film inductive (TFI)-are a totally different design from ferrite or MIG heads. They are so named because of how they are manufactured. TF heads are made using a photolithographic process similar to how processors are made. This is the same technique used to make modern thin film platter media, which bears the same name; see here for more details on this technology. In this design, developed during the 1960s but not deployed until 1979, the iron core of earlier heads, large, bulky and imprecise, is done away entirely. A substrate wafer is coated with a very thin layer of alloy material in specific patterns. This produces a very small, precise head whose characteristics can be carefully controlled, and allows the bulky ferrite head design to be completely eliminated. Thin film heads are capable of being used on much higher-density drives and with much smaller floating heights than the older technologies. They were used in many PC hard disk drives in the late 1980s to mid 1990s, usually in the 100 to 1000 MB capacity range. As hard disk areal densities increased, however, thin film heads soon reached their design limits. They were eventually replaced by magneto-resistive (MR) heads. (4) Magneto- Resistive Heads: Magneto-resistive materials have recently been employed in magnetic disk recording read heads. Such materials experience a change in their resistance in the presence of magnetic flux. A change of two percent in the resistance is typical. However, the change in resistance is not linear and a magnetic bias is required to move the center point of operation into a small area that is linear. Such bias is typically provided by a small permanent magnet positioned nearby. Magneto-resistive (MR) heads are easily saturated by ordinary disk recording levels and for this reason, hard disks recorded by ordinary inductive read/write (R/W) heads are not always compatible with MR head disk drives. In fixed disk drives this is not a problem because the read and write heads and hard disk are part of a permanent set. In removable hard disk drives this can be a significant problem area. MR heads typically have narrow pickup widths. Within a comparatively widely-written data track, the narrow read width is not a problem. In fact, some extra degree of disk run out and track misregistration can be tolerated as the MR head can be allowed to wander within a data track radially without producing a concomitant read-amplitude variation.

Hard Disk Spindle Motor: The spindle motor, also sometimes called the spindle shaft, is responsible for turning the hard disk platters, allowing the hard drive to operate. The spindle motor is sort of a "work horse" of the hard disk. It's not flashy, but it must provide stable, reliable and consistent turning power for thousands of hours of often continuous use, to allow the hard disk to function properly. In fact, many drive failures are actually failures with the spindle motor, not the data storage systems. For many years hard disks all spun at the same speed. In the interests of performance, manufacturers have been steadily ratcheting up their products' spin speeds over the last few years. These higher-speed spindles often have issues related to the amount of heat and vibration they generate. The increased performance and also the new potential issues related to the spindle motor have given it renewed attention in the last few years. Operation: First, the motor must be of high quality, so it can run for thousands of hours, and tolerate thousands of start and stop cycles, without failing. Second, it must be run smoothly and with a minimum of vibration, due to the tight tolerances of the platters and heads inside the drive. Third, it must not generate excessive amounts of heat or noise. Fourth, it should not draw too much power. And finally, it must have its speed managed so that it turns at the proper speed.

Hard Disk Drive Specifications: The specifications associated with hard disk drive are: Seek Time: Seek time is one of the three delays associated with reading or writing data on a computer's disk drive, and somewhat similar for CD or DVD drives. The others are rotational delay and transfer time, and their sum is access time. In order to read or write data in a particular place on the disk, the read/write head of the disk needs to be physically moved to the correct place. This process is known as seeking, and the time it takes for the head to move to the right place is the seek time. Seek time for a given disk varies depending on how far the head's destination is from its origin at the time of each read or write instruction; usually one discusses a disk's average seek time. It is of three types as follows:  Average: As discussed, this is meant to represent an average seek time from one random track (cylinder) to any other. This is the most common seek time metric, and is usually 8 to 10 ms, though older drives had much higher numbers, and top-of-the-line SCSI drives are now down to as low as 4 ms!  Track-to-Track: This is the amount of time that is required to seek between adjacent tracks. This is similar in concept (but not exactly the same as) the track switch time and is usually around 1 ms. (Incidentally, getting this figure without at least two significant digits is pretty meaningless; don't accept "1 ms" for an answer, get the number after the decimal point! Otherwise every drive will probably round off to "1 ms".)  Full Stroke: This number is the amount of time to seek the entire width of the disk, from the innermost track to the outermost. This is of course the largest number, typically being in the 15 to 20 ms range. In some ways, combining

this number with the average seek time represents the way the drive will behave when it is close to being full. Latency: Latency is the amount of time that takes for the platter to spin, bringing the sector to the right position. Access Time: Access time is the metric that represents the composite of all the other specifications reflecting random performance positioning in the hard disk. As such, it is the best figure for assessing overall positioning performance, and you'd expect it to be the specification most used by hard disk manufacturers and enthusiasts alike. Depending on your level of cynicism then, you will either be very surprised, or not surprised much at all, to learn that it is rarely even discussed. Ironically, in the world of CD-ROMs and other optical storage it is the figure that is universally used for comparing positioning speed. I am really not sure why this discrepancy exists. Perhaps the problem is that access time is really a derived figure, comprised of the other positioning performance specifications. The most common definition is: Access Time = Command Overhead Time + Seek Time + Settle Time + Latency. Track Switch Time: The track switch time, also called cylinder switch time, measures in milliseconds the amount of time required to move the read/write heads from one cylinder to an adjacent one. This is a mechanical process that involves using the actuator to physically move the read/write heads. Track switch time is really a special case of seek time, where the seek is being done to an adjacent track. See in that description for some more information, some of which is relevant to track switch time as well. Track switch time is reasonably important because switches to adjacent tracks occur much more frequently than random seeks, when processing larger files. Even during a continuous transfer of large size, the heads must be moved from track to track. An average cylinder on even a modern high-density disk contains less than 1 megabyte of data, which means that a multi-megabyte read or write will involve many cylinder switches. Head Switch Time: Each cylinder contains a number of tracks, each accessible by one of the heads on the drive (one head per surface). To improve efficiency, the drive will normally use all of the tracks in a cylinder before going to the next cylinder when doing a sequential read or write; this saves the time required to physically move the heads to a new cylinder. Switching between heads is a purely electronic process instead of a mechanical one. However, switching between heads within a cylinder still requires a certain amount of time, called the head switch time. This is usually less than the track switch time, and is usually on the order of 1 to 2 milliseconds. (Seems kind of slow for an electronic process, doesn't it? The reason is that this time includes all of the overhead of the switch as well; it is all of the time that passes between when the read stops on one head and when it actually starts again on the next one.) Rotational Speed: It specifies the speed at which the disk platters spin, in revolutions per minute. faster speed means reduces latency and faster internal data rates. Internal Data Rates: It is the speed at which data is transferred between platters and the buffer.

Areal Density: Specifies the amount of data that can be stored on the drive, in BPSI. As mentioned earlier, areal density of a drive is the product of the tpi-multiply by a track’s bpi. So those was all about specifications os hard disk drive.

Sizes: There are various sizes for hard disk drives:  In recent years the hard drive market has benefited from technology advances which allows for larger capacity drives with falling prices. This benefits the users and allows for additional drives to be used either as a main disk or many choosing to add a disk purely for backup purposes.  It is still possible to buy small 20-30GB drives, but for literally £10 more you can purchase a drive with upwards of 60-80GB.  We would suggest a drive of around 120GB for most users as this will allow for plenty of capacity for everyday use and for disk hungry tasks. You can of course divide or partition the drive this allows for backups to be copied from one partition to the other and allows for easier maintenance and organisation of files.

Types: There are various types of hard disk drives as follows:  IDE has long been the standard to allow ATA devices to work together in PCs. There have been several different versions of ATA since it was first introduced in the mid 1980's. The most common that we will mention are ATA 33, ATA 66, ATA 100, and ATA 133. External devices are growing in popularity these typically connect via a USB or FireWire connection.  The IDE/ATA standard is the most common interface also known as PATA. There have been several different versions of ATA since it was first introduced in the mid 1980's. They all focus on the theoretical transfer rate via the interface between the drive and the motherboard.  The connectors are a bank of pins which are at risk to damage. Also the configuration in older systems had to be setup via Jumpers or Bridges. More modern systems could detect via the BIOS using the cable select feature.  Serial ATA (SATA) as mentioned already PATA has been around for many years and as a result of other areas of PC architecture evolving so to have a bottleneck for transferring data quicker between the systems buses and the hard drives. SATA with its first standard supports transfer rates measured at 150 in relation to the previous standards.This standard will allow for faster rates to emerge in coming years.  SATA does away with large ribbon cables and instead uses a thin cable with a small connector which can easily be clipped on to the connector. Bridges have been removed as the drive is automatically detected by the system or separate SCSI card.

Hard disk drives do fail, and protecting your data with regular backups is extremely important. If you do suffers a hard disk drive failure, you might be able to take advantage of the drive manufacture’s warranty, even if the warranty on your computer has expired.

Steps To HDD Troubleshooting: 1. Physical connectivity—Is the drive receiving power? Is it plugged into the PC by a correctly connected ribbon cable? For IDE drives, are its jumpers set correctly? Or with SCSI drives, are its SCSI termination and ID set correctly? 2. BIOS setup—Does the BIOS see the drive? 3. Viruses—Does the drive contain any boot sector viruses I need to remove before continuing? 4. Partitioning—Does FDISK find a valid partition on the drive? Is it active? 5. Formatting—Is the drive formatted using a file system that the OS can recognize? 6. Drive errors—Is a physical or logical drive error causing read/write problems on the drive? 7. Operating system—Does your OS have a feature that checks the status of each drive on your system? If so, what is that status?