How do Hard Disk Drives Work? 💻💿🛠

Imagine every picture you took over the past year,  all saved within the area of a single dot of ink from a ballpoint pen. Well, this is approximately  how compact data is stored inside a hard disk drive, and in this video, we’re going to open one  up and see how an entire library worth of books is able to fit within the surface of this metal disk. We’ll start by opening up this hard drive and detailing the components inside.

After that, we’ll  dive into exactly how the drive stores data using the read and write head, and in the process, we’ll  look at the tracks, sectors, and magnetic domains of the metal disk. Finally, we’ll explore some of  the latest advances that enable over a terabit of data to fit within every square inch of the  disk. This video is sponsored by PCBWay; more on them later, but for now let’s jump right in.

On the inside of this drive, we find a variety of components. Here’s the disk or platter that  stores all the data, and, depending on the storage capacity of the drive, might be multiple  platters tall. The disk is composed of an aluminum magnesium alloy with multiple coatings of other  alloys, but the magnetic, functional layer is this 120-nanometer thin layer of a cobalt chromium  tantalum alloy which has small magnetic domains or regions whose direction can be manipulated via  external magnetic fields.

The platter is mounted on a spindle which spins at a speed of 7200  rpm using a brushless DC motor at its center. Next, there’s a head stack assembly, with  one arm above and one arm below each disc, and with a slider and a read/write head at the  end of each arm. The slider is uniquely designed such that it catches the airflow generated by  the ludicrously fast-spinning disk and uses the air flow to float or fly the read/write head so  that it’s only 15 nanometers or about 100 atoms away from the surface of the disk.

For reference,  here’s the thickness of a sheet of aluminum foil. Because the arm assembly flies on top of  the spinning disk, it’s only brought over the surface when it’s at full speed, and when  the disk is not spinning, the arm assembly is parked to the side on a small piece of plastic. Back here a voice coil motor composed of a coil of wire and two strong neodymium magnets above  and below is used to move the entire arm stack assembly.

When electric current is run through  the coil it creates an electromagnet which is influenced by the neodymium magnets, thus  generating a force that causes the arm to move across the disk. When a reverse current is sent  through the voice coil, the arm is forced in the opposite direction, thereby enabling control of  the exact position of the read/write head within 30 or so nanometers. Additionally, the magnets and  voice coil make a rather strong motor that enable the lightweight arm stack assembly and read/write  head to move back and forth to different tracks across the platter up to 20 times a second  and to make small adjustments incredibly fast.

In order to connect to the read/write head, a  flexible ribbon of wires is routed along the side of the arm and down to this connector which  feeds signals to the outside of the hard drive enclosure and to the printed circuit board or PCB.  On the PCB we have the main processor as well as a DRAM chip, which is used as a scratchpad for  the processor and a buffer for the incoming and outgoing data. Additionally mounted on the PCB  is a chip for controlling the voice coil and brushless DC spindle motor, and then on the edge  of the PCB is a SATA connector which connects to the motherboard for communications and a separate  connector which goes to the power supply.

Additional important components are the gasket  that seals the disk from the exterior environment and two filters that catch any stray dust  particles. These filters are necessary since the read/write heads are just 15 nanometers away  from the platter, and a single dust particle can be up to 10,000 nanometers large and could  cause major damage if it were to collide with the 7,200-rpm disk. Now that we’ve looked through  many of these components, let’s see how they work.

To begin, the disk is divided into concentric  circles of tracks. The latest hard drives can have more than 500,000 tracks on just one side.  These tracks are then divided into sectors, and in each sector is a preamble or synchronization  zone which tells the read/write head the exact speed of the spinning disk and the length of each  bit of data.

The next part of the sector is the address which helps the read/write head know which  track and sector it’s currently positioned over. After that we have the actual data that’s stored,  typically 4 kilobytes of data per sector. Next is an area for an error correcting code, or ECC,  which is used to verify that the data stored in the block is accurately written and properly read,  and finally there’s a gap between this sector and the next which allows the read/write head some  tolerance when writing the contents of a block.

Now let’s zoom in on the read/write head  and the disk to see exactly how data is written and read. Writing data to the disk  is done by manipulating the direction of magnetization of a localized region or domain  of the cobalt-chromium-tantalum layer in the disk and forcing the region to be magnetized  in the up direction or the down direction. This tiny magnetic domain or region is around 90  by 100 by 125 nanometers, and when magnetized, all the atoms will have their even tinier magnetic  north/south poles pointing in the same direction.

In order to magnetize a single domain, which  is equivalent to writing a single bit of data, a current is applied to a coil of wire at  the back of the write head, thus creating a strong magnetic field back here. The magnetic  field is channeled through the write head and focused into a small point at the tip and then  jumps across the 15-nanometer air gap and into the disk. When the focused magnetic field passes  into a single domain of cobalt-chromium-tantalum, all these atoms are forced to align  their tiny atomic magnetic fields with the applied magnetic field from the  write head, thus turning the small domain or region into a permanent magnet.

The key is  that even when the write head is moved away, the direction of the magnetic domains in this  layer of the disk is maintained for years, and they emit a permanent magnetic field which can  be repeatedly sensed by the read head every time you read out the data stored in this sector. That  is, of course, until the computer and write head rewrite a new bit of data to the domain by either  flipping the direction or keeping it the same. Let’s explore how we read data from the disk.

Thus  far we’ve been showing domains as pointing up as a binary 1 and pointing down as a 0. While this  is conceptually simple, it isn’t actually the case. Rather the read head is designed to detect  the changes in orientation from magnetic domains pointing in one direction and then the adjacent  domain pointing in the opposite direction.

This is because emitted magnetic fields from adjacent  regions that switch their orientations are much stronger than the emitted field from just a  single domain pointing one direction or the other. Therefore, each change in magnetic domain  pointing in one direction to the opposite direction is assigned a 1, and an absence  of a transition from one domain to the next is assigned a 0. Therefore, the write head would  record a binary sequence of 0011 0010 like this.

Or a sequence of 1101 1110 like this, where the  1’s are changes, and the 0’s are lack of changes. So then, what’s inside the read head that  detects these magnetic fields? Well, inside is a multilayer conductive material composed  of alternating layers of ferromagnetic and non-magnetic materials.

This multilayer material  has a property called giant magnetoresistance or GMR, and, put simply, it’s a material that  changes its resistivity depending on the strength of magnetic fields that pass through it.  Therefore, using GMR it’s a simple matter of just measuring the resistivity, and when there’s a low  resistivity that means there are strong magnetic fields below the read head resulting from a change  in domain orientation and it’s a 1, and when there’s high resistivity and no change it’s a 0. However, this poses the problem of how a string of dozens of non-changing domains can result  in an ambiguous number of zeroes.

To fix this, in each sector the preamble is simply a  set of alternating domains and is used to set the size of each domain, and then  the error correcting code at the back of the sector is used to ensure no data is lost. Next, we’re going to explore some advancements in hard disk drive technology that improve the  areal density, which is the number of bits that can fit within a given area. In this graph,  you can see how areal density has increased by over 50 million times throughout the past 60  years.

However, perhaps more important is that the cost to store trillions of bits of data has  dropped by over 100 million times. Just imagine, if we were to time travel this hard drive  back to the 1960s, it would be worth over 4 billion dollars, and now in 2022 it costs  less than 40 dollars and is far faster and more reliable than the disk drives from the 60s! Similar to the trend in hard drives, you can buy inexpensive, and yet incredibly reliable printed  circuit boards from our sponsor PCBWay.

Whether you’re prototyping your next project, or ready to  mass produce thousands of your finalized devices, PCBWay can quickly manufacture your PCBs with  competitive prices and impeccable standards. Additionally, if you don’t want to spend weeks  soldering all the components to every board, PCBWay provides PCB assembly services where  they populate and solder the components to the PCB for you. They’ll send pictures throughout  the assembly process, and you can work directly with PCBWay’s engineers to provide programming  and testing protocols.

The next time you have a project and want to save both time and  money, consider using PCBWay to manufacture and populate all your Printed Circuit Boards.  Thank you PCBWay for sponsoring our channel and supporting engineering education. Check out  PCBWay using the link in the description below.

Let’s now return to hard drives and see  some of the advances that allow this disk to store terabytes of data. First, around 2010 the  orientation of the domain was switched from being horizontal to vertical, and with it the write and  read heads also had to change their orientation. This change in orientation is due to the fact  that as a magnetic domain shrinks in volume, it becomes more easily affected by temperature. 

So, by changing the orientation to vertical, the domains or magnetic regions can utilize the  depth of the material while continuing to shrink the area on the disk that each domain takes up. The next advancement we’re going to look into is called Shingled Magnetic Recording or SMR,  which started being commercially available around 2020. Before shingled magnetic recording,  hard drives typically used the technique, Classic Magnetic Recording or CMR, where the  tracks of data are 90 nanometers wide and have guard bands on either side of the track.

However, with shingled recording, the tracks are written to partially overlap with  previously written tracks with no guard bands to separate each track. Thus, we can fit many more  tracks and much more data into a given area. Note that the read head is much smaller than the  write head, and as a result one shingled track can be reliably read at a time.

The issue  however is that if you write over a track, and the upper track of shingled data is still  good or valid data, the drive will first have to read and store that valid data in the DRAM  buffer, and then write both the lower track of new data and the upper track of valid data. And,  as a result, the buffering and the extra read and write steps can result in a loss of performance. The third advancement we’re going to discuss is heat assisted magnetic recording or HAMR which  is not yet commercially available.

Essentially, this technology utilizes a small, focused laser to  heat the region that is actively being written to. By heating the domain, the magnetic region can  be more easily influenced or coerced to orient in a particular direction. This is necessary  because both the write head and the localized electromagnetic field are incredibly small  and, by making the magnetic region more easily coerced using focused heat, we can continue  to shrink the size of the magnetic domain.

That’s pretty much it for how Hard disks work.  Thank you to all of our Patreon and YouTube Membership Sponsors for helping to make this  video. This is Branch Education, and we create 3D animations that dive deep into the technology  that drives our modern world.

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