Hard Drive 101

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Where do you begin when talking about hard drives? The ports? The data? No, wait I have it. The history of the hard drive and all its glory!!!! Nah, I’m just yanking your chain. I wouldn’t do that to you nor could I.

Disclaimer: I’m not an engineer. If you expect an engineering type analysis of a HDD and its magnetic properties… go talk to someone smarter than I. Also, everything is my original work except for cited material and the contributions of other authors [cited as well].
The hard drive is the slowest part of your computer in most cases. A controversial case is game load times but more on that later. Let’s dig into the innerds!

This is your mission should you choose to accept it:

Mission: Go!

Let’s start very basic. A hard drive is a data storage device that uses the properties of magnets to store information. The information is written in 1’s and 0’s (binary) on platters. Platters are stacked one on top of another and attached to the spindle motor. The number of platters varies from hard drive to hard drive and can number from 1 to 5 typically. Sometimes even ½ a platter is all you have. To get the total space on a hard drive multiply the number of platters by the space on each platter. I can see the wheels turning in your head, yes I can see into your brain. You are thinking “Why doesn’t the size of the hard drive on the box match what I see in Windows?” Marketing gurus will tell you it is due to formatting but they’d be wrong and you’d be misinformed. The real reason that the manufacturer’s size and the size in windows don’t match boils down to different definitions of a kilobyte.

Counteracting Marketing Brainwashers

According to Windows a kilobyte = 1024 bytes of information, a megabyte = 1024 kilobytes, etc. However, according to hard drive manufacturers a kilobyte = 1000 bytes, a megabyte = 1000 kilobytes, etc. See the problem? Knowing this you can predict the usable space on a hard drive. If you want to go through the full calculation take the hard drive size in GB (according to the manufacturer) and multiply it by 1,000,000,000 to convert it into bytes. Then divide that number by 1024^3 to get GB according to Windows. Want a shortcut? Take the size in GB according to the manufacturer and multiply it by 93.13%.

There you have it. Marketing is brainwashing everyone into thinking the loss in space is due to formatting but you know better.

How Much Information Can One Platter Hold?

Areal density is a measure of how much information can be stored on a square inch of a platter. Areal density is typically measured in Gigabits per square inch. Do not get this confused with Gigabytes, they are two different metrics. A byte is equal to 8 bits. A bit is the absolute smallest piece of information on a computer it is either 1 or 0, On or Off. Toshiba has announced a 1.8” hard drive (think Ipod size) that can store 333 Gigabits per square inch on a single platter. To sort of put this in perspective a 1 Terabyte drive from Seagate (1000GB = 1TB according to hard drive manufacturers) boasts only 180 Gigabits per square inch which yields 250GB (big B means byte, little b means bit) per platter on a 3.5” form factor. 333 Gb per inch translates into a 120GB Ipod.

Pushing The Data Density Envelope

Have you noticed how rapidly hard drive sizes seem to be mushrooming? Well that is perpendicular media recording for ya. Traditionally (i.e. before PMR) the bits on the platters surface were oriented parallel to the plan of the platter. If you can imagine a person is lying on their side on the ground with the person represents a bit on a platter and the ground represents the platter. To increase areal density hard drive manufactures brought the individual bits (or people in our illustration) closer and closer together in order to get more bits in the same physical space. However there is a limit to how close the bits could get. If they got too close the super paramagnetic effect occurrs. That is just a big fancy word for saying the individual bits would flip polarity. A bit that was positive (1 in binary) would pull a 180 and become negative (0 in binary). That’s bad… very bad. That would mean instant data corruption. To counteract that phenomenon hard drive manufacturers started placing the bits on end. So in our illustration the person would no longer be laying on their side, instead they would be standing on their feet thus perpendicular to the plane of the platter (the ground). This allows each bit to get a lot cozier with other bits without going psychotic and switching its charge. This recording feature is called perpendicular media recording (PMR). PMR made its debut in smaller drives initially where space was extremely limited (laptops, ipods, travel drives, etc) but has migrated to hit virtually every size hard drive. The high density drives (750GB+) are all PMR based but in the lower end spectrum you can still find traditional media recording as well as PMR. Is there really a difference? Yes and no. Yes there is a difference (the ways to measure that will be talked about later) but is the difference noticeable? Probably not. Given the same price I’d go with a PMR drive but if it costs another $10 just save your money and buy another drink at the bar this weekend.

Hard drive platter removed from its housing.

Read/Write Heads

You know that little clicking sound you hear when your computer is loading or starting up? That is the actuator arm moving back and forth across the platter which holds the bits of information you want to read. On the end of the actuator arm are tiny little devices designed to read the polarity of the bits on the platter. The heads (as I will collectively refer to them) use an electrical current applied to a small coil at the end of the head to create a small magnetic field to write to the platter. Conversely, the hard drive applies a magnetic field to the coil to allow an electrical current to flow in order to read information stored on the platters. These get very technical and the exact electromagnetic properties involved are beyond the scope of this article (and me unfortunately). These heads hover extremely close to the platter, within millimeters (or maybe less) of the platter surface.

Actuator arm when removed.

You ever wonder why people are so paranoid about getting dust in their hard drive or why hard manufacturers produce their products in probably one of the cleanest environments on the planet? Well they want to avoid head crashes. Don’t make it any more complicated than what it sounds like because it is that simple. A head crash is when the read/write head crashes into the platter. Ok, that sounds way more dramatic than it actually is. Al

Ways to Evaluate a Hard Drive

Ok this is where I insert my only recommendation: Check a hard drives warranty and ease of exchange. Enthusiasts usually never keep products long enough to exercise their legal right under product warranties but this is a key area where warranties rule. Most people probably wouldn’t be bothered to replace comonents like a motherboard, a PSU, video cards, etc. but replacing a hard drive is childs play (as is a video card but you get the point). Thus, if you are setting your mom up with a new computer you might want to give thought to how easy it is to RMA a drive in case of failure…. Because you will have one fail. Don’t tell me you haven’t had to deal with it yet because you will. Plain and simple.

Anyways, back on point.

Sequential Transfer Rate

There are very different ways to measure a hard drives performance depending on your intended usage. Server side drives are benchmarked on an entirely different set of critereon than consumer level drives. I believe there are 3 main criterion that you should judge a hard drive by (besides price). First is Sequential Transfer Rate (STR). STR is one of the most frequently quoted performance measurements. STR measures the transfer rate (data rate) the hard drive is capable of sustaining across the platters. It is always a negatively sloped line in the case of traditional hard drives (Solid State Disks are another story). The reason the slope is negative is due to angular velocity. In simple terms the outside edge of the hard drive is spinning at a faster rate than the inside part of the disk because it is further from the center. Now because hard drive manufacturers are smart people they engineered hard drives to be filled up from the outside edge in. Meaning as more data is written to the hard drive the closer the data gets to the center of the platter where the angular velocity is lower and thus the transfer rate is slower which leads to a negative slope.

This is a graph from HDTach, one of the more prominent Hard drive benching apps (all be it limited one).

Seek Time/Rotational Latency

The next factor in evaluating a hard drive is seek time. In order to read data from the platters the heads have to physically positioned over the right part of the platter. The time it takes for the actuator arm to move the heads from their current position to the necessary position is the seek time. Seek time is typically an average value. A lower seek time is better because the hard drive actuator arm is able to move the heads to the correct position faster allowing for a more responsive feel. Seek time isn’t usually a big deal when doing large file transfers (that’s where STR comes into play). Where seek times come into play is when a system needs to read lots of smaller files scattered scross different areas of the platter [more on ways to reduce this effort later]. Seek times are directly related to spindle speed. The higher the spindle speed the lower the seek time.

What happens is the actuator arm positions the heads over the part of the platter that it wants to begin reading the cluster and once the heads get to the right position the drive must wait for the correct portion of the platter to pass underneath the head. This platter delay is referred to as rotational latency.
The rotational latency is also inversely proportional to the spindle speed. Here is a table of rotational latency by spindle speed:

[code]4200    RPM    5400    RPM    7200    RPM    10000    RPM
7.1    MS    5.6    MS    4.2    MS    3.0    MS[/code]Notice the direct correlation between spindle speed and rotational latency. Now we get into some basic math to break down the average seek time into it’s true seek time. Remember that the head must be positioned before the platter area pass under the head thus if we subtract the rotational latency from the average seek time quoted we get true seek time. So if a drive is quoted as having 8ms avg seek time and is a 10,000RPM drive the true seek time is 5ms (8ms seek time less the 3ms rotational latency). This is of course talking averages, average seek minus rotational latency gives you avg true seek time. Each and every read/write command is different so you cannot talk in absolutes.

Areal Density

Yes, I’ve talked about it before and no I’m not beating a dead horse (I prefer live ones). The point to be made here is that two drives that are both 500GB and similar in everyway except one drive has 5x 100GB platters and the other has 2x 250GB platters will perform differently. The hard drive with 250GB platters will have higher transfer rates than the one with 100GB platters because each pass under the heads passes more information due to higher areal density. This is why the Samsung 1TB drives (3x 333GB platters, 7200RPM) can match and even exceed the Raptors (10,000RPM, 150GB) sequential transfer rate even though the Raptor has a significant edge in spindle speed. The only problem with this criterion is that it isn’t exactly a published spec. In order to find out the areal density you will have to search for a review online. Typically within the same product line (i.e. Seagate 7200.11 series) the manufacturer only varies the number of platters so if you find a different size hard drive but it’s the same series you should be able to extrapolate the platter count. Now this is NOT always true but generally is.

Maintenance and Setup

Hard Drive Formatting – Low Level & Zero Fill Formatting By: DaSickNinja

Hard drives sometimes need to be formatted, especially when data is corrupted or the drive needs to be sold. In order to do such various techniques must be used to insure that the drive contains no data. There are two (actually three) types of formatting performable on hard disk drives (HDD). The most commonly used techniques fall under the category of High Level Formatting (HLF). These techniques are quick and easy to perform, but data is still easily recoverable. To truly remove data and ensure that the drive is empty, low or base level formatting (LLF) tools are used.

Low Level Formatting By: DaSickNinja

Low Level Formatting (LLF) is the process of outlining the positions of the tracks and sectors, and writing the control structures that define where the tracks and sectors are. Tracks are the concentric circles located on the surface of each platter, shaped similarly to the rings of a tree. Each track is divided into sectors with 16 sectors being the average count for a typical 5.25 HDD platter. Because of the use of platter geometry and structure, low level formatting is often called a “true” formatting operation because it actually creates the physical format that defines where data is stored on a hard disk.

Because of the type of operation that a true low level format entails, the first time a LLF is performed on a disk, the platters will start empty. That is the last time the platters will be data free. If LLF is done while data is on the drive, with the exception of forensic data recovery, the information is permanently erased.

On older drives, LLF often needs to be performed because of the thermal expansion problems associated with using stepper motor actuators. As time progresses, the heating and cooling cycles of hard drive use will actually cause the positioning of the tracks and sectors to move from where the heads would expect them to be. This causes data errors and as such, requires a LLF to rewrite the tracks in the new positions that the stepper motor moved the heads to. Modern P/S-ATA and SCSI hard drives however use voice coil actuators to move the actuator arms and heads. Newer disks also use many complex internal structures, including zoned bit recording to put more sectors on the outer tracks than the inner ones, and embedded servo data to control the head actuator. They also transparently map out bad sectors. Due to this complexity, and sector realignment techniques all modern hard disks are low-level formatted at the factory for the life of the drive. There is a way for the PC to do an LLF on a modern P/S-ATA or SCSI hard disk, but in actuality, there is no reason to try to do so.

Zero Fill Formatting By: DaSickNinja

Note: Zero fill techniques are actually a form of reinitialization. Because of the confusion caused by the present ambiguity in the term "high-level format", various drive manufacturers have actually described reinitialization software as LLF utilities on their web sites.

When most users today talk about "low-level formatting" a drive, what they really mean is performing a zero fill. Zero fill formatting is literally what the name implies, by writing a zero byte to every addressable location on the disk, the data previously held on the drive is "erased"

This procedure will restore a mechanically functional drive to the condition it was in when received from the factory. You’d usually resort to this method when a HDD is so corrupted the OS cannot recover the data on its own. Boot sector viruses for example can be hard to eradicate without resorting to low-level intervention. Since the zero-fill cleans all programs and data off the drive it will get rid of almost any data-related problem on the drive, such as viruses, corrupted partitions and the like. Like burning your house to get rid of rats, you wipe everything off the drive.
Usually, zero fill utilities include the following features as part of the software package.

  • Drive Recognition Test – Lets you test to see if the software can detect the drive. This is the first step in ensuring that there are no mechanical errors with the HDD.
  • Display Drive Details – Tells you detailed information about the drive, such as its exact model number, firmware revision level, date of manufacture, etc.
  • Test For Errors – Analyzes the entire surface of the hard disk, looking for problem areas (bad sectors) and instructing the drive controller to remap them.
  • Zero-Fill – Wipes off all data on the drive by filling every sector with zeroes. Normally a test for errors (stated above) is done concurrently.

This type of utility can also be used to hide bad sectors by instructing the HDD to remap the bad sectors to its “spares”. If this needs to be done with regularity though, it’s a large possibility that the drives’ reliability is suspect.

Zero Fill Procedure By: DaSickNinja

Always use the zero fill utility provided by the hard drive maker, i.e. Seagate drive – Seagate DiscWizard Utility. These steps are only for Seagate drives. Check your drives manufacturer for the correct steps to reinitialize your drive.

1. Download utility from Seagate Technology – DiscWizard
2. Create bootable disk
3. Boot from the


Question: I saw the actuator arm picture and was just going to ask if any drives had independently moving arms…but you answered it!

Any explanation why we dont have..say…Raptors with multiple moving arms?

Answer: Good question. My guess is cost.

The actuator controller and arms would have to be more complex as would the control circuitry. It would only improve latency and read/write rates if the clusters in question were on different platters. If the information was on the same platter you’re hosed, not even multiple moving actuators could help you there. Also with more complexity comes a higher risk of failure which in the server environment is unacceptable so the multiple movement actuator arms are probably pretty 1337 quality and if they dumb them down the increase in the failure right is higher than the pay off.

Raptor is already at a significant premium compared to what you get so you may see a multiple moving actuator be added eventually if WD wants to keep up the brand.

The only real time you’d see this become an advantage is in multi instruction environments. Think multiple users hitting a DB (like XCPUs) with lots of little queries all over the disk. Given that set of circumstances a multi movement actuator would be very helpful but for you and me on a daily basis… you just can’t justify the cost.

Let’s say I want to set up some partitions on an existing drive. What utilities can I use?

Answer:]If you mean resizing an existing partition, its best to do such only when you have unallocated space on that physical drive. One good way to do such would be a with the Windows install CD/DVD as a fresh installation. The better way would be with a partition editor, such as Partition Magic or Acronis Partition expert. For those that are into open source methods, GParted works excellent and is light weight, since it runs as a LiveCD.


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