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. Watch another Branch
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