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SEE OUR COMPUTER GLOSSARY!!

A HDD or Hard Disk Drive is the device that holds all of the information on your computer, including the operating system (“O/S” - such as Windows, Linux or Apple). 

How it works: Traditionally, a hard disk drive is comprised of one or more hard metal disks (“platters”) and an actuator “arm” (sometimes referred to as the read-and-write head) for each that is magnetically maneuvered across the spinning disk in order to access the digital information stored on the drive .  For a 240 Mb Western Digital drive, for example, each HDD has three magnetic platters at 80Gb of storage each, with its own actuator arm, for a total of 240Gb storage.  The nearly weightless suspension arm hovers a hair (i.e. a nanometer) above the magnetic disk which spins at either 5400 or 7,200 revolutions per minute, server drives as fast as 15,000 rpm.  The outer edge of a typical disk, therefore, travels at 100 miles per hour.  The arm holds a recording head the size of a pepper flake, which sits above the disk at a height measured in nanometers - less than the ridges of a fingerprint. 

Each disk drive contains more than 200 parts, most of them designed for specific models.  The data is retained on the disk even when the HDD is off (as opposed to cache or RAM, where the data is not held in memory once the power is turned off.  See MEMORY).  An orange (usually) flat ribbon cable connects the mechanical portion of the drive to the logic board, a circuit board that contains a tiny, low power processor controller chip located underneath the spinning disks, which routes the data read from the disks via the actuator arm and sends it to the computer’s motherboard through the IDE, SCSI or SATA cable connected to the drive through the connectors at the edge of the HDD (along with the appropriate power cable).  (See the diagram below for a graphic look at all this.)

Solid State Drives work much the same way as traditional drives, except there are nothing but chips and absolutely no moving parts. The chips are usually NAND flash memory as well as a (considerably faster) controller chip to route the data to the PCs motherboard much the same way as the physical type HDDs. In addition, most SSDs use a few hundred megabytes of RAM as a buffer between the NAND and the PC.   Click HERE for the full story about SSDs.

Hard drives organize their user stored data into regions of the magnetic media called blocks.  These blocks are, in turn, located in regions of the disk called a sector. (Sometimes, unfortunately, these terms are used interchangeably.) These drive sectors are located along the concentric tracks on the surface of the disk.  Within each sector is also located a servo and error correction information that allows the magnetic heads to stay on track so that they can read and write data, and for the electronic channel to recover the data, if necessary, without errors.  The sectors also contain information that allows the file system to reassemble the data blocks into computer files.  While a sector is the smallest unit that can be accessed on a HDD, a cluster is a slightly larger unit that is used to organize and identify the files on a disk.   Most files take up several clusters of disk space.  Each cluster therefore has a unique ID, which enables the drive to locate all the clusters on a disk, as they are rarely contiguous and are “fragmented” (hence the periodic necessity for drive “defragmentation”.) 

See the diagram immediately below for a graphic representation of this structure.  As you can see, data is stored on the surface of the platter in sectors and tracks.  Tracks (sometimes called “cylinders”) are concentric circles (one is selected in red [“A”]), while sectors are shown as (purple [“B”]) pie-shaped wedges radiating out from the center to the edge of the disk. Newer drives store bits in overlapping tracks (known as “shingled magnetic recording”) like shingles on a home’s roof.  There are a fixed number of sectors per track.  Tracks are small:  On a typical drive, about 60 to 100 tracks can fit within the width of a human hair.  Each sector contains a fixed number of bytes, usually 512, which comprise a block, which is a single cell in that sector [“C”].  There are about 3 trillion bits per square inch on the platter of a common hard disk.  Further, at some point either the operating system or the drive firmware may group together one or more sectors into clusters (shown in green [“D”]). 

hdd drive sectors

PHOTO: SOFTPEDIA

 

However, before a computer can write data to a hard drive or any other media, it must be  formatted, both at the low-level and the high-level.  The low level formatting lays out the tracks and sectors discussed above.  This is usually done at the factory, because it is based on a calculation for each drive, using “heads X sectors X tracks”.  Each block usually holds 512 bytes of data.   But the HDD still can’t be used by the computer without high-level formatting, sometimes called partitioning. This process, usually done through the computer’s operating system, prepares the file storage structure (like FAT or NTFS or ReFS) into the drive’s sectors, so that data can then be written. Each type of formatting has its own rules and limitations.  Until this is done, creating a partition table on the disk, the computer’s BIOS (or UEFI, which allows more than 4 partitions per drive) can’t know whether the HDD is to be used as a single disk or whether it will be split into one of more logical disks, as well as which part of the disk the boot sector may be located on.  

After reading and writing many files to the HDD, some clusters may remain labeled as being used although they contain no data.  These are called lost clusters and can be fixed using Windows Scan Disk or the Mac Utility programs; also defragmentation can help free up additional hdd space. [Now you know what the report is talking about when you run the Windows ScanDisk utility.]

In addition, when installed on a computer, Windows drives may be configured as either basic or dynamic disks.  [The concept of a “disk” should not be confused with a “drive”.  The drive is the hardware, while the disk is the partition on the drive, formatted with a valid file system such as NTFS.] A basic disk uses primary and extended partitions and logical drives to organize data.  A dynamic disk, on the other hand, can contain a very large number of volumes (a/k/a partitions).  Most Windows PCs are basic disks, as they are simple to manage and more complexity is unnecessary.  Dynamic disks are used more for servers, where multiple disks within a computer or across several computers are used to manage data, such as for RAID applications, as they can be spanned, striped and mirrored to create data reliability.  Dynamic disks can only be used on the computer on which they are created, while basic disks may be used on other computers than the one on which it was created. 

See also the definitions in the Glossary for the boot sector and the MBR (“Master Boot Record”), typically the first sector in the first partition in the drive which starts the operating system.  [Although newer 64-bit drives and operating systems like Windows 8 now use the UEFI instead of the boot sector MBR and, if you have an Apple computer, you use APM).] 

For the past 30 years, the size of the sectors on the hard drive was limited to 512 bytes for most OSs, including Windows and the ATA interface.  Recently, Linux and other open-source developers have led a standards effort to create a 4,096 byte sector standard often referred to as the 4K sector.  This 8x increase in sector size would increase formatting and operating efficiency of the newer, larger drives, especially by allowing for a more robust ECC (Error Correcting Code), critical to creating high area drive densities, and is probably long overdue.

HDDs come in different capacities, speeds and connections (e.g. IDE, SCSI, SATA; disk spin speeds starting at 5400rpm, then 7200 rpm [most common] then 10,000, 12,500 or even 15,000rpm for server drives; capacities from 40Gbs to 10 terabytes, the most common these days about 500Gb; see bits & bytes).  Possibly a more important measure of a drive’s speed, however, is its seek time.  That is, the time which it actually takes a hard drive controller to locate a specific piece of stored data on the disk.  It is measured in nanoseconds or one-thousandth of a second, a range of 7 - 9ns (nanoseconds, or one billionth of a second)  being common for most hard drives.  Alternate measurements of drive speed include track-to-track time (the time, usually in milliseconds (thousands of a second), it takes the read/write head to search or seek between adjacent tracks, usually 2 - 4ms) and full stroke (the amount of time required to seek the entire disk drive, also measured in milliseconds, usually below 10ms).  The measurement begins when the computer’s operating system makes a request to the drive controller firmware to locate certain data.  Then, the controller actuates the read/write head to the position where that data is stored on the disk.  If the head must switch between tracks on the disk, the actuator must move the access arm (see diagram below), increasing the seek time, and which is also dependent on the starting point for the arm when it completed the previous seek command.  Optical drives (CDs/DVDs, floppy disks, and large or mobile drives) have a slower seek time because of their larger head construction.  Average time for CD/DVDs is 65ms, 75ms for DVD-RAMs.  Of course, these speeds are stated as “average” times, as actual times may vary due to transfer time (“data rate,” or the actual amount of time that it takes for data to be read/written) and rotational delay (“latency,” the actual amount of time it takes for the hard disc to rotate to the position for the read/write head to access the data), and also because there is no industry standard for measurement.

Hard drives can be internal, i.e. connected directly to the motherboard via parallel, SATA, eSATA, SCSI or other cable connections.  They can also be connected externally via USB, SATA, eSATA or other cables that can allow them to be physically removed after whatever backup or storage activities have been completed (it’s not a good idea to leave such drives always on, it can wear them out).  And, as the capacities for external flash drives have increased exponentially, they are also a good substitute for external memory or even backup.

HARD DRIVE MAINTENANCE: Aside from keeping the drive in a well-ventilated and clean environment, and one without extreme noise (yes, it’s been documented that high noise environments can cause spinning drive platters to misread data or slow performance), there are some utilities that keep it in tip-top shape.  Your hard drive’s “health” falls into two categories - physical and logical.  Physical health are monitored and checked by the Self-Monitoring, Analysis and Reporting Technology (a/k/a “SMART”) subsystem built into almost every modern hard drive. SMART checks for possible defects in the hardware itself and attempts to avert any upcoming failure.  SMART works automatically.  Logical health of the drive (i.e. the O/S and files written on the drive) involves manual testing.  The Windows O/S has built in software tools for this purpose.  From XP on, chkdsk (see check disk) serves that purpose.  To get to this utility, in Windows Explorer, right click the appropriate drive, then select Properties, click on the Tools tab and, under Error-checking, click the “Check Now” button.  That’s the basic version.  For more comprehensive testing, run chkdsk.exe from the command line and you will see various switches that you can select for advanced options.  [I usually select “chkdsk c: /f” to find and repair disk errors.]

MORE LINKS: For a more detailed discussion about the relationship between electricity, magnetism and binary computers, click HERE.  Also, read about Solid State Drives (SSDs) and Hybrid drives, which have both benefits and drawbacks (like never defragmenting them!). Read those definitions before buying and installing an SSD!!  Click HERE for some interesting history about hard drives by PC World Magazine.  Also, for those businesses that don’t want to protect their data by migrating it to the CLOUD, there are also lines of fire and flood resistant hard drives (offering, for example, 30  minutes of fire protection up to 1550 degrees Fahrenheit and 72 hours of immersion protection in up to 10 feet of water).  See ioSafe drives for between $250 - $600. Also, Virtual Hard DrivesAnd see FAQ #29 and the discussion at the SSD glossary definition about how to “defragment” or  wipeyour drive if necessary.

Interesting features can be available:  For example, if you have an SSD drive, Secure Drives makes a drive equipped with a built-in self-destruct feature, activated to automatically self-destruct when you send an SMS message to its built in cell phone, or several other options.

Finally, drives can get quite HOT.  Particularly those on laptops.  Make sure that they’re properly ventilated (and dust free), otherwise they may crash.  That’s why many laptops must be put on cooling “pads” if they’re used all day - the heat will cause shut-downs.

See the LINKS page for a short history and deconstructed photos of an actual HDD like the one below:

HARD DISK DRIVES DECONSTRUCTED

DISK DRIVES:  THE PAST

RAMAC2

The first hard drive was shipped by IBM in the RAMAC 305 system, introduced on 9/4/56.  It was the size of 2 refrigerators, used 50 24” platters spinning at 1200 rpm and cost $50,000 for all of 5Mb of storage.

old tape drive

Uniservo tape drives as used on the Univac 1 computers in the 1950s

PHOTOS: WIKIPEDIA

old magnetic drum drive

Back in the 1950s magnetic drums like this were used to read and write data

For photos of all types of storage media for computers, click HERE.

DISK DRIVES:  THE PRESENT

HDD Photo

An example of the newer SATA hard drives which hold about 500Gb [IDE (PATA) drives are being phased out, used mostly in older computers]

DRIVE JUMPERS, PINS AND CABLES

classic connectors

Photo Credit: Windows Secrets

SATA connections

From left, back of drive (either HDD or CD/DVD) on IDE (PATA) drive:  (1) power pins, (2)jumper pins(3) drive cable pins

Photo Credit: Newegg Learning

SATA Power and Drive Connections

ON IDE DRIVES, MULTIPLE DRIVES MUST BE SELECTED BY JUMPER:

MASTER (SOLE DRIVE)

SLAVE (SECONDARY DRIVE)

CABLE SELECT (COMPUTER DETERMINES ORDER)

pin labels

Photo Credit: Windows Secrets

DISK DRIVES:  THE FUTURE

ssd

Solid State Drive (“SSD”) can hold terabytes of data without any mechanical activity

Photo Credits:  Tech Republic

Hybrid SSD

Hybrid Solid State Drive (Seagate Momentus line) combines SSD with high speed spinning drive for performance advantages at lower cost

[Photos of drive cables and connectors are shown elsewhere on this site.]

DO ONLY IDE DRIVES HAVE JUMPERS?

NO.  SCSI DRIVES (BOTH 50 (LESS THAN 18Gb) OR 68 PIN OR SCA) DO, NUMBERED A0, A1, A2 AND A3, AS WELL AS A4 FOR SCSI WIDE DRIVES (UP TO 15 DEVICES)

  HOWEVER, SATA and SSD DRIVES MUST EACH HAVE THEIR OWN CABLE, WHILE IDE AND SCSI DRIVES CAN SHARE A CABLE, THEREFORE THERE ARE NO CABLE SELECTION JUMPERS ON SATA & SSD DRIVES [although some SATA drives do have jumpers to limit the speed if the motherboard is not capable of adjusting automatically, and some SSDs have jumpers to short the drive for erasure]

WHY do hard drives fail?

If you’ve followed the discussion above, you realize that your hard drive is a mechanical device, much like a motorized record turntable.  It is not a matter of “if” but of “when” it will fail.  Many factors can cause failure, often a combination of factors, among them:

- Age and use (mechanical degradation).  If there is contact between the heads and the magnetic coating on the platter, it can wear “thin” and develop surface defects, degrading performance.  Luckily, in anticipation of these factors, drive manufacturers build in “spare sectors” which are unused platter surfaces which are held in reserve against future damage and which, through internal software, detect trouble areas and start error correction routings which automatically remap the damaged sectors, move the data to a spare sector and lock out the damaged sector, all without user knowledge or intervention.  Drives also have a technology named S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) that also automatically monitors disk drives  (both Apple and PCs) to warn about impending disk failure.

- Physical damage, e.g. excessive vibration, heat buildup from poor ventilation.  Also, sudden changes in temperature or even humidity.

- Static electricity (damages the drives circuit board) or electrical surge.

- Excessive Heat and Dust.  Just like all other parts of a computer, heat and dust (which can create heat on components) can shorten the life of a hard disk drive.  However, tests (by Blackblaze, for example) have shown that keeping drives cooler than their recommended operating temperature do not decrease their failure rate.  And the failure rate is increased only by excessive heat, not the ordinary heat generated by the hard drives.

- Abuse (floating only a micron above the platters where the data is stored, the drive heads can easily be offset by a sudden jarring movement or ongoing vibration)  Many laptops, such as Toshiba Tecra brand laptops for example, have special sensor software built in to the computer to “park” the drive head automatically if it senses vibration, protecting the drive disk from damage.

- Even software can cause failures, including bad firmware or malware.

WHEN do hard drives fail?

Of course, there’s no hard and fast rule here, but there are some guidelines.  It’s the old “Infancy>Old Age>S**T happens in between rule that governs most computer hardware.  That is, most drives fail either almost immediately (due to latent manufacturing defects) or after a long period of usage (they just wear out).   In between, drives can fail due to usage problems (dropped laptop, hit by power surge, bad power supply, or even viruses).  So, they can possibly fail at any time.  (Although, if it’s between infancy and old age, you can usually point to one of the problems discussed above.)  You should always be prepared, protecting your data by backing it up as if a crash is imminent.  That way, you’ll have no nasty surprises.  For more discussion, see below.

HOW do you know when your hard drive is failing?  Here are a few signals - -

1.  If you hear a “ticking” or a “clicking” (the so-called “click of death”) coming from your drive.

2.  If you get a boot message that says anything about SMART.

3.  If you get the Microsoft Blue Screen of Death, either upon boot or otherwise.

4.  If you get a boot message that your machine can’t find the operating system.

5.  You have to re-boot more than once to access the operating system because it freezes or locks-up when trying to load.

6.  Saved files mysteriously disappear.

7.  You experience performance degradation, especially when saving and opening files.

What do you do when your hard drive is failing?

This is the time to call for professional help.  You CAN’T fix this yourself.  Call Computer Coach, but FIRST:

SHUT DOWN THE MACHINE. 

DO NOT START IT UP AGAIN (AS IT MAY BE THE LAST TIME IT CAN BE STARTED AND YOU MAY LOSE YOUR DATA IF IT COMPLETELY CRASHES, IF IT HASN’T ALREADY).

DO NOT USE RECOVERY SOFTWARE OR CHKDSK.  CHKDSK, FOR EXAMPLE, IS DESIGNED TO ASSURE THAT YOUR DRIVE IS WORKING PROPERLY WITH LITTLE REGARD FOR YOUR DATA, AND RUNNING IT COULD RESULT IN DATA LOSS.  RECOVERY SOFTWARE WILL NOT WORK ON A PHYSICALLY DAMAGED DRIVE, AND MAY MAKE THE SITUATION MUCH WORSE.  CLICK HERE FOR MORE ABOUT DRIVE RESTORATION.

CALL FOR HELP AS QUICKLY AS POSSIBLE. DATA RECOVERY AFTER A COMPLETE CRASH CAN BE TIME CONSUMING AND EXPENSIVE.  YOU CAN’T FIX THIS YOURSELF.

FAQ:  What’s that little hole on your HDD do that says “Do not cover this hole”? Is it kind of like that tag on your mattress that says “do not remove under penalty of law?”  Sort of, but this really is directed at the end user, not the seller:  First of all, it’ll invalidate your warranty, because you’ve tampered with the HDD.  The generally accepted explanation is that it is a “relief valve” which has to stay open in order to equalize the pressure between the inside of the drive and the outside.  As the drive spins, it forces air out through the hole and creates a difference in pressure which has to be stabilized, otherwise it may be damaged.  I don’t recommend this, but if you ever happen to be holding a drive when it starts up or powers down, you’ll feel a tremendous “pull” from the centrifugal force.

FAQ:  What do we mean when we say that “hard disk drive prices are at the lowest level in decades”?  Well, back in 1990, when Windows 3.0 was first introduced, a 20Mb hard drive cost about $900!  That translates to about $1600 in today’s dollars.  Fast forward to today, when a 1Tb drive goes for less than $100!  Not only that, but today’s drives are faster, last longer, are lighter and are more impervious to physical damage than the old ones.  In sum, they’re a bargain!

FAQ:  Are all hard drives created equal?  No.  Some are better than others and last longer.  2013 test results of over 27,000 different sizes, types and models of hard drives by Backblaze, a testing company, shows that Hitachi drives crush competing models from Seagate and Western Digital when it comes to reliability, measured by MTBF.  MTBF should usually result in a “bathtub curve” with higher failure rates initially (“infant mortality”) due to defects and also at the end-of-life (generally somewhere between  100,000  and one million hours (as they age).   But the study doesn’t mean as much as you might think:  The manner of testing, which takes place in cages with lots of vibration and other hostile conditions can cause failures that are less likely to happen on desktop machines.  But it’s a good overall measure of quality.  But you don’t have to run out and buy Hitachi drives just yet, either:  Hitachi sold its drive business to Western Digital in 2012, and WD sold its 3.5 inch drive division to Toshiba, and its too soon to tell whether this will have any impact on their reliability.  See the test results below:

hard drive failures
hard drive AFR

5/2014 UPDATE: Backblaze published its test results of over 27,000 consumer grade hard drives, and the results rank Hitachi Deskstar models first (0.8% annual failure rate for the 5K3000 3TB model) and the Seagate Barracuda 7200 models last (with a whopping 25.4% annual failure rate of its 1.5TB drives).  Click HERE for the full report...

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FAQ:  Who is General Failure and why is he reading my hard disk? I believe he’s the brother of General Insurance, the cartoon mustached general in the TV car insurance ads. But he doesn’t travel with a penguin.

But why does General Insurance travel with a penguin?  Click HERE for the explanation.

General Insurance

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