Basic Hard Disk Drive Components

Many types of hard disk drives are on the market, but nearly all share the same basic physical components. Some differences might exist in the quality of these components (and in the quality of the materials used to make them), but the operational characteristics of most drives are similar. The basic components of a typical hard disk drive are as follows:

Disk platters

Read/write heads

Head actuator mechanism

Spindle motor (inside platter hub)

Logic board (controller or printed circuit board)

Cables and connectors

Configuration items (such as jumpers or switches)
 

Hard Disk Platters (Disks)

A hard disk drive has one or more platters, or disks.

Most hard disk drives have two or more platters, although some of the smaller drives used in portable systems have only one. The number of platters a drive can have is limited by the drive's vertical physical size. The maximum number of platters I have seen in any 3.5-inch drive is 12; however, most drives have six or fewer.

Platters have traditionally been made from an aluminum/magnesium alloy, which provides both strength and light weight. However, manufacturers' desire for higher and higher densities and smaller drives has led to the use of platters made of glass (or, more technically, a glass-ceramic composite).

One such material, produced by the Dow Corning Corporation, is called MemCor. MemCor is made of a glass-ceramic composite that resists cracking better than pure glass. Glass platters offer greater rigidity than metal (because metal can be bent and glass cannot) and can therefore be machined to one-half the thickness of conventional aluminum disks—sometimes less. Glass platters are also much more thermally stable than aluminum platters, which means they do not expand or contract very much with changes in temperature. Several hard disk drives made by companies such as IBM, Seagate, Toshiba, Areal Technology, and Maxtor currently use glass or glass-ceramic platters. In fact, Hitachi Global Storage Technologies (Hitachi and IBM's joint hard disk venture) is designing all new drives with only glass platters. For most other manufacturers as well, glass disks will probably replace the standard aluminum/magnesium substrate over the next few years.

Recording Media
No matter which substrate is used, the platters are covered with a thin layer of a magnetically retentive substance, called the medium, on which magnetic information is stored. Three popular types of magnetic media are used on hard disk platters:

Oxide media

Thin-film media

AFC (antiferromagnetically coupled) media

Oxide Media
The oxide medium is made of various compounds, containing iron oxide as the active ingredient. The magnetic layer is created on the disk by coating the aluminum platter with a syrup containing iron-oxide particles. This syrup is spread across the disk by spinning the platters at a high speed; centrifugal force causes the material to flow from the center of the platter to the outside, creating an even coating of the material on the platter. The surface is then cured and polished. Finally, a layer of material that protects and lubricates the surface is added and burnished smooth. The oxide coating is normally about 30 millionths of an inch thick. If you could peer into a drive with oxide-coated platters, you would see that the platters are brownish or amber.

As drive density increases, the magnetic medium needs to be thinner and more perfectly formed. The capabilities of oxide coatings have been exceeded by most higher-capacity drives. Because the oxide medium is very soft, disks that use it are subject to head-crash damage if the drive is jolted during operation. Most older drives, especially those sold as low-end models, use oxide media on the drive platters. Oxide media, which have been used since 1955, remained popular because of their relatively low cost and ease of application. Today, however, very few drives use oxide media.

Thin-Film Media
The thin-film medium is thinner, harder, and more perfectly formed than the oxide medium. Thin film was developed as a high-performance medium that enabled a new generation of drives to have lower head-floating heights, which in turn made increases in drive density possible. Originally, thin-film media were used only in higher-capacity or higher-quality drive systems, but today, virtually all drives use thin-film media.

The thin-film medium is aptly named. The coating is much thinner than can be achieved by the oxide-coating method. Thin-film media are also known as plated or sputtered media because of the various processes used to deposit the thin film on the platters.

Thin-film-plated media are manufactured by depositing the magnetic medium on the disk with an electroplating mechanism, in much the same way that chrome plating is deposited on the bumper of a car. The aluminum/magnesium or glass platter is immersed in a series of chemical baths that coat the platter with several layers of metallic film. The magnetic medium layer itself is a cobalt alloy about 1 µ-inch thick.

Thin-film sputtered media are created by first coating the aluminum platters with a layer of nickel phosphorus and then applying the cobalt-alloy magnetic material in a continuous vacuum-deposition process called sputtering. This process deposits magnetic layers as thin as 1 µ-inch or less on the disk, in a fashion similar to the way that silicon wafers are coated with metallic films in the semiconductor industry. The same sputtering technique is then used again to lay down an extremely hard, 1 µ-inch protective carbon coating. The need for a near-perfect vacuum makes sputtering the most expensive of the processes described here.

The surface of a sputtered platter contains magnetic layers as thin as 1 µ-inch. Because this surface also is very smooth, the head can float more closely to the disk surface than was possible previously. Floating heights as small as 10nm (nanometers, or about 0.4 µ-inch) above the surface are possible. When the head is closer to the platter, the density of the magnetic flux transitions can be increased to provide greater storage capacity. Additionally, the increased intensity of the magnetic field during a closer-proximity read provides the higher signal amplitudes needed for good signal-to-noise performance.

Both the sputtering and plating processes result in a very thin, hard film of magnetic medium on the platters. Because the thin-film medium is so hard, it has a better chance of surviving contact with the heads at high speed. In fact, modern thin-film media are virtually uncrashable. If you could open a drive to peek at the platters, you would see that platters coated with the thin-film medium look like mirrors.

AFC Media
The latest advancement in drive media is called antiferromagnetically coupled (AFC) media, which is designed to allow densities to be pushed beyond previous limits. Anytime density is increased, the magnetic layer on the platters must be made thinner and thinner. Areal density (tracks per inch times bits per inch) has increased in hard drives to the point where the grains in the magnetic layer used to store data are becoming so small that they become unstable over time, causing data storage to become unreliable. This is referred to as the superparamagnetic limit, which has been determined to be between 30 and 50Gbit/sq. in. Drives today have already reached 35Gbit/sq. in., which means the superparamagnetic limit is now becoming a factor in drive designs.

AFC media consists of two magnetic layers separated by a very thin 3-atom (6 angstrom) film layer of the element ruthenium. IBM has coined the term "pixie dust" to refer to this ultra-thin ruthenium layer. This sandwich produces an antiferromagnetic coupling of the top and bottom magnetic layers, which causes the apparent magnetic thickness of the entire structure to be the difference between the top and bottom magnetic layers. This allows the use of physically thicker magnetic layers with more stable, larger grains to function as if they were really a single layer that was much thinner overall.

IBM has introduced AFC media into several drives, starting with the 2.5-inch Travelstar 30GN series of notebook drives introduced in 2001, the first drives on the market to use AFC media. In addition, IBM has introduced AFC media in desktop 3.5-inch drives starting with the Deskstar 120 GXP. I expect other manufacturers to introduce AFC media into their drives as well. The use of AFC media is expected to allow areal densities to be extended to 100Gbit/sq. in. and beyond.

Read/Write Heads

A hard disk drive usually has one read/write head for each platter surface (meaning that each platter has two sets of read/write heads—one for the top side and one for the bottom side). These heads are connected, or ganged, on a single movement mechanism. The heads, therefore, move across the platters in unison.

Mechanically, read/write heads are simple. Each head is on an actuator arm that is spring-loaded to force the head into contact with a platter. Few people realize that each platter actually is "squeezed" by the heads above and below it. If you could open a drive safely and lift the top head with your finger, the head would snap back down into the platter when you released it. If you could pull down on one of the heads below a platter, the spring tension would cause it to snap back up into the platter when you released it.

When the drive is at rest, the heads are forced into direct contact with the platters by spring tension, but when the drive is spinning at full speed, air pressure develops below the heads and lifts them off the surface of the platter. On a drive spinning at full speed, the distance between the heads and the platter can be anywhere from 0.5 to 5 µ-inch or more in a modern drive.

In the early 1960s, hard disk drive recording heads operated at floating heights as large as 200–300 µ-inch; today's drive heads are designed to float as low as 10nm (nanometers) or 0.4 µ-inch above the surface of the disk. To support higher densities in future drives, the physical separation between the head and disk is expected to drop even further, such that on some drives there will even be contact with the platter surface. New media and head designs will be required to make full or partial contact recording possible.
 

Automatic Head Parking

When you power off a hard disk drive using a CSS (contact start stop) design, the spring tension in each head arm pulls the heads into contact with the platters. The drive is designed to sustain thousands of takeoffs and landings, but it is wise to ensure that the landings occur at a spot on the platter that contains no data. Older drives required manual head parking; you had to run a program that positioned the drive heads to a landing zone, usually the innermost cylinder, before turning the system off. Modern drives automatically park the heads, so park programs are no longer necessary.

Some amount of abrasion occurs during the landing and takeoff process, removing just a "micro puff" from the magnetic medium—but if the drive is jarred during the landing or takeoff process, real damage can occur. Newer drives that use load/unload designs incorporate a ramp positioned outside the outer surface of the platters to prevent any contact between the heads and platters, even if the drive is powered off. Load/unload drives automatically park the heads on the ramp when the drive is powered off.

One benefit of using a voice coil actuator is automatic head parking. In a drive that has a voice coil actuator, the heads are positioned and held by magnetic force. When the power to the drive is removed, the magnetic field that holds the heads stationary over a particular cylinder dissipates, enabling the head rack to skitter across the drive surface and potentially cause damage. In the voice coil design, the head rack is attached to a weak spring at one end and a head stop at the other end. When the system is powered on, the spring is overcome by the magnetic force of the positioner. When the drive is powered off, however, the spring gently drags the head rack to a park-and-lock position before the drive slows down and the heads land. On some drives, you could actually hear the "ting…ting…ting…ting" sound as the heads literally bounce-park themselves, driven by this spring.

On a drive with a voice coil actuator, you activate the parking mechanism by turning off the computer; you do not need to run a program to park or retract the heads. In the event of a power outage, the heads park themselves automatically. (The drives unpark automatically when the system is powered on.)  

SMART

SMART (Self-Monitoring, Analysis, and Reporting Technology) is an industry standard providing failure prediction for disk drives. When SMART is enabled for a given drive, the drive monitors predetermined attributes that are susceptible to or indicative of drive degradation. Based on changes in the monitored attributes, a failure prediction can be made. If a failure is deemed likely to occur, SMART makes a status report available so the system BIOS or driver software can notify the user of the impending problems, perhaps enabling the user to back up the data on the drive before any real problems occur.

Predictable failures are the types of failures SMART attempts to detect. These failures result from the gradual degradation of the drive's performance. According to Seagate, 60% of drive failures are mechanical, which is exactly the type of failures SMART is designed to predict.

Of course, not all failures are predictable, and SMART cannot help with unpredictable failures that occur without any advance warning. These can be caused by static electricity, improper handling or sudden shock, or circuit failure, such as thermal-related solder problems or component failure.

SMART originated in technology that was created by IBM in 1992. That year IBM began shipping 3.5-inch hard disk drives equipped with Predictive Failure Analysis (PFA), an IBM-developed technology that periodically measures selected drive attributes and sends a warning message when a predefined threshold is exceeded. IBM turned this technology over to the ANSI organization, and it subsequently became the ANSI-standard SMART protocol for SCSI drives, as defined in the ANSI-SCSI Informational Exception Control (IEC) document X3T10/94-190.

Interest in extending this technology to ATA drives led to the creation of the SMART Working Group in 1995. Besides IBM, other companies represented in the original group were Seagate Technology, Conner Peripherals (now a part of Seagate), Fujitsu, Hewlett-Packard, Maxtor, Quantum, and Western Digital. The SMART specification produced by this group and placed in the public domain covers both ATA and SCSI hard disk drives and can be found in most of the more recently produced drives on the market.

The SMART design of attributes and thresholds is similar in ATA and SCSI environments, but the reporting of information differs.

In an ATA environment, driver software on the system interprets the alarm signal from the drive generated by the SMART "report status" command. The driver polls the drive on a regular basis to check the status of this command and, if it signals imminent failure, sends an alarm to the operating system, where it will be passed on via an error message to the end user. This structure also enables future enhancements, which might allow reporting of information other than drive failure conditions. The system can read and evaluate the attributes and alarms reported in addition to the basic "report status" command.

SCSI drives with SMART communicate a reliability condition only as either good or failing. In a SCSI environment, the failure decision occurs at the disk drive, and the host notifies the user for action. The SCSI specification provides for a sense bit to be flagged if the drive determines that a reliability issue exists. The system then alerts the end user via a message.

The basic requirements for SMART to function in a system are simple. All you need are a SMART-capable hard disk drive and a SMART-aware BIOS or hard disk driver for your particular operating system. If your BIOS does not support SMART, utility programs are available that can support SMART on a given system. These include Norton Disk Doctor from Symantec, EZ-Drive from StorageSoft, and Data Advisor from Ontrack Data International.

Note that traditional disk diagnostics, such as Scandisk and Norton Disk Doctor, work only on the data sectors of the disk surface and do not monitor all the drive functions that are monitored by SMART. Most modern disk drives keep spare sectors available to use as substitutes for sectors that have errors. When one of these spares is reallocated, the drive reports the activity to the SMART counter but still looks completely "defect free" to a surface analysis utility, such as Scandisk.

Drives with SMART monitor a variety of attributes that vary from one manufacturer to another. Attributes are selected by the device manufacturer based on their capability to contribute to the prediction of degrading or fault conditions for that particular drive. Most drive manufacturers consider the specific set of attributes being used and the identity of those attributes as vendor specific and proprietary.

Some drives monitor the floating height of the head above the magnetic media. If this height changes from a nominal figure, the drive could fail. Other drives can monitor different attributes, such as ECC (error-correction code) circuitry that indicates whether soft errors are occurring when reading or writing data. Some of the attributes monitored on various drives include the following:

Head floating height

Data throughput performance

Spin-up time

Reallocated (spared) sector count

Seek error rate

Seek time performance

Drive spin-up retry count

Drive calibration retry count

Each attribute has a threshold limit that is used to determine the existence of a degrading or fault condition. These thresholds are set by the drive manufacturer, can vary among manufacturers and models, and cannot be changed.

The basic requirements for SMART to function in a system are simple. All you need is a SMART-capable hard disk drive and a SMART-aware BIOS or hard disk driver for your particular operating system. If your BIOS does not support SMART, utility programs are available that can support SMART on a given system. These include Norton Utilities from Symantec, EZ Drive from StorageSoft, and Data Advisor from Ontrack.

Any drives reporting a SMART failure should be considered likely to fail at any time. Of course, you should back up the data on such a drive immediately, and you might consider replacing the drive before any actual data loss occurs. When sufficient changes occur in the monitored attributes to trigger a SMART alert, the drive sends an alert message via an ATA or a SCSI command (depending on the type of hard disk drive you have) to the hard disk driver in the system BIOS, which then forwards the message to the operating system. The operating system then displays a warning message as follows:

Immediately back up your data and replace your hard disk drive. A failure may be imminent.

The message might contain additional information, such as which physical device initiated the alert, a list of the logical drives (partitions) that correspond to the physical device, and even the type, manufacturer, and serial number of the device.

The first thing to do when you receive such an alert is to heed the warning and back up all the data on the drive. It also is wise to back up to new media and not overwrite any previous good backups you might have, just in case the drive fails before the backup is complete.

After backing up your data, what should you do? SMART warnings can be caused by an external source and might not actually indicate that the drive itself is going to fail. For example, environmental changes, such as high or low ambient temperatures, can trigger a SMART alert, as can excessive vibration in the drive caused by an external source. Additionally, electrical interference from motors or other devices on the same circuit as your PC can induce these alerts.

If the alert was not caused by an external source, a drive replacement might be indicated. If the drive is under warranty, contact the vendor and ask whether they will replace it. If no further alerts occur, the problem might have been an anomaly, and you might not need to replace the drive. If you receive further alerts, replacing the drive is recommended. If you can connect both the new and existing (failing) drive to the same system, you might be able to copy the entire contents of the existing drive to the new one, saving you from having to install or reload all the applications and data from your backup.

Performance

When you select a hard disk drive, one of the important features you should consider is the performance (speed) of the drive. Hard drives can have a wide range of performance capabilities. As is true of many things, one of the best indicators of a drive's relative performance is its price. An old saying from the automobile-racing industry is appropriate here: "Speed costs money. How fast do you want to go?"

Normally the speed of a disk drive is measured in several ways:

Interface (external) transfer rate

Media (internal) transfer rates

Average access time

Average Seek Time

Average seek time, normally measured in milliseconds (ms), is the average amount of time it takes to move the heads from one cylinder to another a random distance away. One way to measure this specification is to run many random track-seek operations and then divide the timed results by the number of seeks performed. This method provides an average time for a single seek.

The standard method used by many drive manufacturers when reporting average seek times is to measure the time it takes the heads to move across one-third of the total cylinders. Average seek time depends only on the drive itself; the type of interface or controller has little effect on this specification. The average seek rating is primarily a gauge of the capabilities of the head actuator mechanism.

Transfer Rates

The transfer rate is probably more important to overall system performance than any other statistic, but it is also one of the most misunderstood specifications. The problem stems from the fact that several transfer rates can be specified for a given drive; however, the most important of these is usually overlooked.

A great deal of confusion arises from the fact that drive manufacturers can report up to seven different transfer rates for a given drive. Perhaps the least important of these (but the one people seem to focus on the most) is the raw interface transfer rate, which for the 2.5-inch ATA drives used in portable systems is 100MBps. Unfortunately, few people seem to realize that the drives actually read and write data much slower than that. The most important transfer rate specifications are the media (or internal) transfer rates, which express how fast a drive can actually read or write data. Media transfer rates can be expressed as a raw maximum, raw minimum, formatted maximum, formatted minimum, or averages of any of these. Few report the averages, but they can be easily calculated.

The media transfer rate is far more important than the interface transfer rate because it is the true rate at which data can be read from (or written to) the disk. In other words, it tells how fast data can be moved to and from the drive platters (media). It is the rate that any sustained transfer can hope to achieve. This rate will normally be reported as a minimum and maximum figure, although many drive manufacturers report the maximum only.

Media transfer rates have minimum and maximum figures because drives today use zoned recording with fewer sectors per track on the inner cylinders than the outer cylinders. Typically, a drive is divided into 16 or more zones, with the inner zone having about half the sectors per track (and therefore about half the transfer rate) of the outer 0zone. Because the drive spins at a constant rate, data can be read twice as fast from the outer cylinders than from the inner cylinders.

Two primary factors contribute to transfer rate performance: rotational speed and the linear recording density or sector-per-track figures. When two drives with the same number of sectors per track are being compared, the drive that spins more quickly will transfer data more quickly. Likewise, when two drives with identical rotational speeds are being compared, the drive with the higher recording density (more sectors per track) will be faster. A higher-density drive can be faster than one that spins faster—both factors have to be taken into account to know the true score.

To find the transfer specifications for a given drive, look in the data sheet or preferably the documentation or manual for the drive. These can usually be downloaded from the drive manufacturer's Web site. This documentation will often report the maximum and minimum sector-per-track specifications, which—combined with the rotational speed—can be used to calculate true formatted media performance. Note that you would be looking for the true number of physical sectors per track for the outer and inner zones. Be aware that many drives (especially zoned-bit recording drives) are configured with sector translation, so the number of sectors per track reported by the BIOS has little to do with the actual physical characteristics of the drive. You must know the drive's true physical parameters rather than the values the BIOS uses.

When you know the true sector per track (SPT) and rotational speed figures, you can use the following formula to determine the true media data transfer rate in millions of bytes per second (MBps):


Media Transfer Rate (MBps) = SPTx512 bytesxrpm/60 seconds/1,000,000 bytes


For example, the Hitachi/IBM Travelstar 7K60 drive spins at 7,200rpm and has an average of 540 sectors per track. The average media transfer rate for this drive is figured as follows:


540x512x(7,200/60)/1,000,000 = 33.18 MBps


Some drive manufacturers don't give the sector per track values for the outer and inner zones, instead offering only the raw unformatted transfer rates in Mbps (megabits per second). To convert raw megabits per second to formatted megabytes per second in a modern drive, divide the figure by 11. For example, Toshiba reports transfer rates of 373Mbps maximum and 203Mbps minimum for its MK6022GAX 60GB drive. This is an average of 288Mbps, which equates to an average formatted transfer rate of about 26MBps.

As you can see from the table, even though all these drives have a theoretical interface transfer rate of 100MBps, the fastest 2.5-inch drive has an average true media transfer rate of just over 33MBps. As an analogy, think of the drive as a tiny water faucet, and the ATA interface as a huge firehouse connected to the faucet that is being used to fill a swimming pool. No matter how much water can theoretically flow through the hose, you will only be able to fill the pool at the rate the faucet can flow water.

The cache in a drive allows for burst transfers at the full interface rate. In our analogy, the cache is like a bucket that, once filled, can be dumped at full speed into the pool. The only problem is that the bucket is also filled by the faucet, so any data transfer larger than the size of the bucket can proceed only at the rate that the faucet can flow water.

When you study drive specifications, it is true that larger caches and faster interface transfer rates are nice, but in the end, they are limited by the true transfer rate, which is the rate at which data can be read from or written to the actual drive media. In general, the media (also called internal or true) transfer rate is the most important specification for a drive.
 

BIOS Limitations

If your current hard drive is 8GB or smaller, your system might not be able to handle a larger drive without a BIOS upgrade, because many older (pre-1998) BIOSes can't handle drives above the 8.4GB limit, and others (pre-2002) have other limits, such as 137GB. Although most ATA hard drives ship with a setup disk containing a software BIOS substitute such as OnTrack's Disk Manager or Phoenix Technologies' EZ-Drive (Phoenix purchased EZ-Drive creator StorageSoft in January 2002), I don't recommend using a software BIOS replacement. EZ-Drive, Disk Manager, and their OEM offshoots (Drive Guide, MAXBlast, Data Lifeguard, and others) can cause problems if you need to boot from floppy or CD media or if you need to repair the nonstandard master boot record these products use.

If your motherboard ROM BIOS dates before 1998 and is limited to 8.4GB, or dates before 2002 and is limited to 137GB, and you wish to install a larger drive, I recommend you first contact your motherboard (or system) manufacturer to see if an update is available. Virtually all motherboards incorporate a Flash ROM, which allows for easy updates via a utility program.

Operating System Limitations

Newer operating systems such as Windows Me as well as Windows 2000 and XP fortunately don't have any problems with larger drives; however, older operating systems may have limitations when it comes to using large drives.

DOS will generally not recognize drives larger than 8.4GB because those drives are accessed using LBA (logical block addressing), and DOS versions 6.x and lower only use CHS (cylinder, head, sector) addressing.

Windows 95 has a 32GB hard disk capacity limit, and there is no way around it other than upgrading to Windows 98 or a newer version. Additionally, the retail or upgrade versions of Windows 95 (also called Windows 95 OSR 1 or Windows 95a) are further limited to using only the FAT16 (16-bit file allocation table) file system, which carries a maximum partition size limitation of 2GB. This means if you had a 30GB drive you would be forced to divide it into 15 2GB partitions, each appearing as a separate drive letter (drives C: through Q: in this example). Windows 95B and 95C can use the FAT32 file system, which allows partition sizes up to 2TB (terabytes). Note that due to internal limitations, no version of FDISK can create partitions larger than 512MB.

Windows 98 supports large drives, but a bug in the FDISK program included with Windows 98 reduces the reported drive capacity by 64GB for drives over that capacity. The solution is an updated version of FDISK that can be downloaded from Microsoft. Another bug appears in the FORMAT command with Windows 98. If you run FORMAT from a command prompt on a partition over 64GB, the size isn't reported correctly, although the entire partition will be formatted.

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