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Drive Anatomy: Hard Drives

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It is magnetic. He is electric. He is photonic. No, this is not a new superhero trio from the Marvel universe. It's about storing our precious digital data. We need to store them somewhere, securely and stably, so that we can have access to them and change them in no time. Forget about Iron Man and Thor - we are talking about hard drives!

So, let's dive into the anatomy of the devices that we use today to store billions of data bits.

You spin me right round, baby


Mechanical drive on hard drives (hard disk drive, HDD) has been the standard storage systems for computers around the world for more than 30 years, but the underlying technology is much older.

The first commercial HDD company IBM released in 1956 , its capacity was as much as 3.75 MB. And in general, over all these years, the general structure of the drive has not changed much. It still has disks that use magnetization to store data, and there are devices for reading / writing this data. The amount of data that can be stored on them has changed , and very much.

In 1987, you could buy a 20 MB HDD for about $ 350; today for the same money14 TB can be bought: 700,000 times the volume.

We’ll look at a device that is not quite this size, but also worthy by modern standards: the 3.5-inch Seagate Barracuda 3 TB HDD, in particular, the ST3000DM001 model , notorious for its high percentage of failures and legal processes caused by this . The drive we are studying is already dead, so it will be more like an autopsy than an anatomy lesson.


The bulk of the hard drive is cast metal. The forces inside the device during active use can be quite serious, so thick metal prevents bending and vibration of the case. Even in tiny 1.8-inch HDDs, metal is used as the case material, but usually they are made not of steel, but of aluminum, because they should be as light as possible.


Turning the drive over, we see a printed circuit board and several connectors. The connector at the top of the board is used for the engine that rotates the disks, and the bottom three (from left to right) are jumper contacts that allow you to configure the drive for certain configurations, the SATA data connector (Serial ATA) and the SATA power connector.


Serial ATA first appeared in 2000. On desktop computers, this is the standard system used to connect drives to the rest of the computer. The format specification has undergone many revisions, and now we are using version 3.4. Our corpse of the hard drive has an older version, but the difference is only in one contact in the power connector.

In data connections, a differentiated signal is used to receive and receive data : contacts A + and A- are used to transfer instructions and data to the hard disk, and contacts B are used to receive these signals. Such use of paired conductors significantly reduces the effect of electrical noise on the signal, that is, the device can work faster.

If we talk about power, we see that the connector has a pair of contacts of each voltage (+3.3, +5 and + 12V); however, most of them are not used because the HDD does not require a lot of power. This particular Seagate model uses less than 10 watts of active load. Contacts marked as PC are used for precharge : this function allows you to remove and connect the hard drive while the computer continues to work (this is called hot swapping ).

Contact with PWDIS tag allows remote resethard drive, but this feature is only supported with SATA 3.3, so in my drive it's just another + 3.3V power line. And the last contact, marked as SSU, simply tells the computer whether the hard drive supports staggered spin up spindles .

Before the computer can use them, the disks inside the device (which we will see soon) must spin up to full speed. But if a lot of hard drives are installed in the machine, then a sudden simultaneous request for power can harm the system. The gradual unwinding of the spindles completely eliminates the possibility of such problems, but you will have to wait a few seconds before getting full access to the HDD.


After removing the printed circuit board, you can see how it connects to the components inside the device. HDDs are not hermetic , with the exception of devices with very large capacities - they use helium instead of air, because it is much less dense and creates less problems in drives with a large number of disks. On the other hand, you should not expose conventional drives to the open environment.

Thanks to the use of such connectors, the number of input points through which dirt and dust can get into the drive is minimized; there is a hole in the metal case (a large white dot in the lower left corner of the image), which allows preserving the environmental pressure inside.


Now that the circuit board is removed, let's see what is inside. There are four main chips:

  • LSI B64002: main controller chip that processes instructions, transfers data streams in and out, error correction, etc.
  • Samsung K4T51163QJ: 64 MB DDR2 SDRAM clocked at 800 MHz used for data caching
  • Smooth MCKXL: controls the engine spinning discs
  • Winbond 25Q40BWS05: 500 KB of serial flash memory used to store drive firmware (a bit like a computer’s BIOS)

PCB components of different HDDs may vary. For large volumes, more cache is required (in the most modern monsters there can be up to 256 MB DDR3), and the main controller chip can be a little more sophisticated in error handling, but in general the differences are not so great.

Opening the drive is simple, just unscrew a few Torx bolts and voila! We are inside ...


Given that it occupies the main part of the device, our attention immediately attracts a large metal circle; It’s easy to understand why drives are called disk drives . Correctly call them plates ; they are made of glass or aluminum and are coated with several layers of various materials. This 3 TB drive has three plates, i.e. 500 GB should be stored on each side of one plate.


The image is quite dusty, such dirty plates do not correspond to the accuracy of design and production necessary for their manufacture. In our HDD example, the aluminum disk itself has a thickness of 0.04 inches (1 mm), but is polished to such an extent that the average height of the surface deviations is less than 0.000001 inches (about 30 nm).

The base layer has a depth of only 0.0004 inches (10 microns) and consists of several layers of materials deposited on the metal. The application is carried out using chemical nickel plating , followed by vacuum deposition , preparing the disk for the basic magnetic materials used to store digital data.

This material is usually a cobalt alloy and is composed of concentric circles, each of which is approximately 0.00001 inches (approximately 250 nm) wide and 0.000001 inches (25 nm) in depth. At the micro level, metal alloys form grains similar to soap bubbles on the surface of the water.

Each grain has its own magnetic field, but it can be converted in a given direction. Grouping such fields results in data bits (0 and 1). If you want to learn more about this topic, then read this document from Yale University. The final coatings are a carbon layer for protection, and then a polymer to reduce contact friction. Together, their thickness is not more than 0.0000005 inches (12 nm).

Soon we will see why the plates should be made with such strict tolerances, but it’s still amazing to realize that for only $ 15 you can become the proud owner of a device manufactured with nanometer precision!

However, let's go back to the HDD itself and see what else is in it.


The metal cover is shown in yellow, which securely fastens the plate to the spindle drive electric motor - the electric drive that rotates the disks. In this HDD, they rotate at a frequency of 7200 rpm (rpm), but in other models they can work more slowly. Slower drives have lower noise and power consumption, but also lower speed, and faster drives can reach speeds of 15,000 rpm.

To reduce the damage caused by dust and moisture in the air, a recirculation filter (green square) is used to collect small particles and hold them inside. The air moved by the rotation of the plates provides a constant flow through the filter. Above the discs and next to the filter there is one of three plate separators: Helps reduce vibration and keep air flow as even as possible.

In the upper left part of the image, one of the two permanent bar magnets is indicated by a blue square. They provide the magnetic field needed to move the component indicated in red. Let's separate these details to see them better.


What looks like a white patch is another filter, only it purifies particles and gases that enter from the outside through the hole we saw above. Metal spikes are levers for moving heads on which are read / write heads of a hard disk. They move with great speed along the surface of the plates (upper and lower).

Check out this video created by The Slow Mo Guys to see how fast they are:


The design does not use something like a stepper motor ; To move the levers along the solenoid, an electric current is conducted at the base of the levers.


In general, they are called voice coils , because they use the same principle that is used in speakers and microphones to move membranes. The current generates a magnetic field around them, which responds to the field created by bar permanent magnets.

Remember that the data tracks are tiny , so the positioning of the levers must be extremely accurate, like everything else in the drive. Some hard drives have multi-stage levers that make small changes in the direction of only one part of the whole lever.

On some hard drives, data tracks overlap. This technology is called tiled magnetic recording. (shingled magnetic recording), and its requirements for accuracy and positioning (that is, to hit constantly at one point) are even stricter.


At the very end of the levers there are very sensitive read / write heads. Our HDD contains 3 plates and 6 heads, and each of them floats above the disk during its rotation. For this, the heads are suspended on ultra-thin strips of metal.

And here we can see why our anatomical specimen died - at least one of the heads loosened, and no matter what caused the initial damage, it also bent one of the levers. The entire component of the head is so small that, as can be seen below, it is very difficult to get a high-quality picture of it with a conventional camera.


However, we can disassemble the individual parts. The gray block is a specially made part called a “slider” : when the disk rotates under it, the air flow creates lift by lifting the head off the surface. And when we say “raises,” we mean a gap with a width of just 0.0000002 inches or less than 5 nm.

A little further, and the heads will not be able to recognize changes in the magnetic fields of the track; if the heads were on the surface, then they would just scratch the coating. That is why it is necessary to filter the air inside the drive casing: dust and moisture on the surface of the disk will simply break the heads.

The tiny metal “pole” at the end of the head helps with overall aerodynamics. However, to see the parts that read and write, we need a better photo.


In this image of another hard drive, readers and writers are under all electrical connections. System Recording is performed TFT inductance (thin film induction, TFI), and the reading of - a tunnel magnetoresistance device (tunneling magnetoresistive device, TMR).

The generated TMR signals are very weak and must be passed through an amplifier before being sent to increase levels. The chip responsible for this is located near the base of the levers in the image below.


As stated in the introduction to the article, the mechanical components and the principle of operation of the hard drive have not changed much over the years. Most of all, the technology of magnetic tracks and read-write heads was improved, creating narrower and denser tracks, which ultimately led to an increase in the volume of stored information.

However, mechanical hard drives have obvious speed limits. It takes time to move the levers to the desired position, and if the data is scattered across different tracks on different plates, the drive will spend quite a few microseconds to search for bits.

Before moving on to another type of drive, let's give indicative speed indicators for a typical HDD. We used the CrystalDiskMark benchmark to evaluate the hard drive.WD 3.5 "5400 RPM 2 TB :


The first two lines indicate the number of MB per second during sequential (long, continuous list) and random (transitions throughout the drive) read and write. The next line shows the IOPS value, that is, the number of I / O operations performed every second. The last line shows the average delay (time in microseconds) between transmitting a read or write operation and receiving data values.

In the general case, we strive to ensure that the values ​​in the first three lines are as large as possible, and in the last line as small as possible. Do not worry about the numbers themselves, we just use them for comparison when we look at another type of drive: a solid state drive.

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