What's Inside an HDD? We Disassemble a Hard Drive Platter by Platter

If you want to know a bit more about HDD construction than just "it's a rectangular box" — this breakdown is for you.

Inside that metal brick is one of the most precise mechanical assemblies found in our computers. Today I'm taking apart a classic 3.5-inch drive piece by piece to show you what each component actually does.

HDA and PCB — Two Worlds in One Enclosure

From the outside, a drive is two things:

• HDA (Head and Disk Assembly) — a metal housing with a hermetically sealed interior. Inside, two independent motors operate (one spins the platters, one positions the heads), along with the platter pack, the arm with heads, and magnets. Factory-filled with filtered air (or helium in newer designs).
• PCB (Printed Circuit Board) — the electronics screwed to the bottom of the HDA.

Both parts are linked at the firmware level. This means you cannot take a PCB from one drive and use it in another, even if the model is identical. Incidentally — the PCB is one of the least frequently damaged components in an HDD.

What's on the PCB

The PCB is a fiberglass laminate (FR-4) with copper traces, and on top sit the electronic components. Looking at a Seagate drive board you can see seven key elements:

1. MCU (Micro Controller Unit) — the drive's processor

   The largest chip on the board. This is the main drive controller. It contains the processor core, read/write channel, SATA interface, and other circuits. It manages all operations and works with memory, VCM, sensors, and other components.

2. DDR / SDRAM — working memory (buffer/cache)

   Fast working memory for the drive controller. Part of the memory acts as a data buffer/cache (read, write, command queuing). The rest is used by the firmware and drive processor during operation.

3. VCM driver — controlling platters and heads

   An integrated circuit controlling the spindle motor and the voice coil positioning the head (VCM). Responsible for precise head movement and stable platter operation.

4. Flash memory (ROM) — firmware and adaptive data

   Stores the startup code, firmware, and adaptive data specific to that drive instance. After power-on, the MCU loads Flash contents into memory. Without correct ROM contents, the drive may not even spin the platters.

   In some drives there is no separate Flash chip — its contents are inside the MCU, e.g. WD 2060-701444-004.

5. Shock sensor — head protection

   Detects shocks and sends a signal to the VCM driver. The number of reported events can be read in SMART parameters.

6. TVS diodes — overvoltage protection

   Protect the board from voltage spikes from the power supply. Two diodes protect the 5V and 12V lines.

7. SATA and power connector — host interface

   Interface for connecting the drive to the computer. Desktop and notebook drives use SATA (data + power). Server versions may also use SAS.

SSHD — Hybrids with NAND

SSHD (Solid State Hybrid Drive) is a classic HDD with an additional NAND chip soldered onto the PCB. NAND stores firmware for fast startup, translator, SMART, and often a cache of frequently used data.

For a data recovery lab, it's a complication: a NAND failure causes the drive not to start despite intact mechanics. If you have a Seagate Rosewood and your drive stopped responding, you may be a victim of the common NAND-related fault.

HDA and the Breather Hole

There's a myth that drives have a vacuum inside. No. Drives breathe. The small hole is the breather hole — it equalizes pressure between the drive interior and the environment. From the inside it is separated by a filter that traps moisture and dust particles.

Helium drives are filled with helium. Helium is less dense than air, allowing more platters and greater capacity. These drives cannot be opened and resealed under home conditions.

Service Area — Firmware on the Platters

Not all drive firmware lives in the Flash memory on the PCB. Most is written on the platters themselves, in an area inaccessible to the user. This area is called the Service Area (SA).

Drive startup sequence:

1. After power is applied, the MCU reads the initialization code from Flash.
2. It starts the motor. The platters spin up to nominal speed (typically 5400 or 7200 RPM).
3. After reaching speed, the arm parks and positions over the service area.
4. The drive loads its firmware modules: model name, serial number, calibration parameters for each head, factory defect list (P-list), runtime defect list (G-list), SMART table, ATA passwords, LBA translator, etc.
5. Only now does the drive appear in BIOS/system.

The translator converts logical LBA addresses from the OS into physical sector locations on the platter. Without it, the drive can't find files. Symptoms: drive shows 0 MB capacity, asks to be formatted, or doesn't respond.

SA failures are a common category in the data recovery lab — one firmware module corrupted, the entire drive is dead. Fixed with specialized equipment (we use PC-3000, MRT).

Platters

Platters are the actual data storage medium. Made of aluminum or glass, polished to a mirror finish, coated with a thin layer of cobalt alloy (typically CoCrPt). This layer stores data as orientations of microscopic magnetic domains.

A 3.5-inch drive holds 1 to 9 platters (the latter only in helium drives). All rotate at the same speed on a common spindle.

• Platter clamp — presses the entire platter pack against the spindle hub at 7200 RPM.
• Spacers — metal rings between platters, maintaining equal gaps.
• Separators/dampers — baffles between platters that dampen turbulence and reduce acoustic vibrations. Aluminum ones also conduct heat away from the HDA interior.

Workshop rule: platters are not removed from the spindle without a special tool (platter stand). One wrong move during disassembly and the pack geometry is ruined.

Heads and Arm (HSA)

The entire head assembly sits on a structure called the HSA (Head Stack Assembly). Components:

• Bearing — precision bearing at the HSA pivot axis.
• Arm — milled from solid aluminum. Stiffer and lighter = faster movement without vibration.
• HGA (Head Gimbal Assembly) — the arm tip holding the actual head, connected through a flexible pivot.
• FPC (Flexible Printed Circuit) — orange flexible tape connecting HSA electronics to the HDA contact pad.
• Voice coil — sits between two neodymium magnets, forming the VCM (Voice Coil Motor). Changing current moves the arm over the appropriate track.

Slider, ABS and the Actual Head

The black block at the end of the HGA is the slider — an aerodynamic wing that keeps the read/write elements at the correct height above the platter.

The write element uses a magnetic field from a tiny coil. The read element is a magnetoresistive sensor (GMR or TMR). The slider's bottom surface has grooves forming the ABS (Air Bearing Surface), maintaining the slider at ~5–10 nanometers above the platter.

This fly height is why a drive must not be opened outside a laminar flow cabinet. A dust particle has a diameter of ~10 micrometers — a thousand times more than the head fly height.

Preamplifier

Inside the HDA, near the base of the HSA, is the preamplifier (preamp). Head signals are very weak (>1 GHz frequencies), so the preamp amplifies them close to the source before sending to the PCB. It also controls which head is active at any moment.

In modern drives the preamp also handles fly height control (AFH in Seagate F3 architecture) and microactuators for precise track following.

Recirculation Filter

The recirculation filter catches microscopic particles generated inside the HDA during operation. Without it, these particles would damage heads and platter surfaces within months.

Magnets and Head Parking

The HSA coil floats between two powerful neodymium magnets — enabling millisecond movement with nanometer precision. The housing also contains HSA stoppers and an actuator latch to keep the arm within its allowed range.

Two Types of Head Parking

CSS (Contact Start/Stop) — heads land on the platter in an inner zone (no data) after power-off.

Load/Unload — heads travel off the platter onto a plastic ramp after power-off.

Why You Must Not Open a Drive at Home

A residential room is roughly ISO Class 9 — several million dust particles per cubic meter. A laminar flow cabinet for opening drives is ISO Class 5 — a few hundred particles per cubic meter.

Opening a drive in a room means contamination many times larger than the head fly height appears under the slider within seconds. After spin-up, these particles damage heads and can destroy the magnetic platter surface.

SMR, CMR and Write Direction

Modern HDDs use PMR (Perpendicular Magnetic Recording). Some consumer "archival" drives use SMR (Shingled Magnetic Recording) — overlapping tracks for greater capacity, at the cost of reliability. SMR drives in a RAID array are a recipe for disaster — rebuild after another drive failure can take weeks.

SED — Hardware-Encrypted Drives

An increasing number of modern HDDs are SED (Self-Encrypting Drive), encrypting data on the fly with AES-128 or AES-256. Without enabling the lock mechanism, the drive decrypts data for any computer and behaves like regular storage.

FAQ

Is there a vacuum inside an HDD?
No. Standard drives contain air exchanged with surroundings through a filtered breather hole. Exception: helium drives.

Why is my drive clicking?
Clicking means it's asking for help. If your data matters, do NOTHING. Take it to a data recovery lab.

Can I swap electronics from another identical drive?
In theory yes, in practice it will only make things worse.

How long does an HDD last?
Statistically 2–8 years. Individual variation is enormous. Backup, backup, backup.

What's the difference between a laptop drive and a desktop drive?
2.5-inch (laptop) drives are smaller, slower (usually 5400 RPM), use load/unload parking, and usually have lower capacity. The rest of the construction is the same.

Does burned electronics mean the data is gone?
In most cases no. If ROM and MCU are intact, we can recover the data. Electronics rarely fail in HDDs.

What is a "donor drive"?
A drive from which working parts are taken for the patient drive. Labs maintain their own stock of donors.