An HDD stores data as magnetic patterns on spinning platter surfaces. An actuator moves the head assembly to the needed track; a read element senses recorded magnetic changes, while a separate write element applies a magnetic field to alter them. Newer recording methods such as heat-assisted magnetic recording (HAMR) modify how writing works, but not every hard drive uses HAMR.
Where the data is stored
Inside a hard disk drive, data is recorded on the magnetic surfaces of rotating platters. The actuator positions the head assembly over a track, and the spinning platter carries the recorded pattern beneath the head. The head operates very close to the surface without serving as a stationary contact point.
Reading and writing are distinct jobs performed by elements in the head assembly: the reader detects magnetic changes in the medium, and the writer changes the medium’s magnetization. The precise design varies across drive generations and manufacturers.
How the head reads data
As a platter rotates, recorded magnetic patterns pass beneath the read element. The reader senses magnetic changes associated with those patterns, and the drive electronics interpret the resulting signal as data. It is a sensing operation: the reader detects what is already recorded rather than changing the platter.
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One example is Seagate’s Gen 8 spintronic reader, which uses tunnelling magnetoresistance (TMR) in a layered magnetic and non-magnetic sensor stack. That is a description of Seagate’s implementation for this generation, not a specification that applies to every HDD. Seagate’s Gen 8 spintronic reader overview explains that design.
How the head writes data
To record or change data, the write element generates a magnetic field that changes the orientation of magnetic regions in the recording medium. Seagate’s technical paper describes the field as a fringing field from the head gap. The writer’s task is therefore different from the reader’s: it actively changes the medium rather than sensing its existing pattern. Seagate’s 2018 technical paper on HAMR and data density discusses the writing process.
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Why recording methods changed
Longitudinal recording and PMR
In longitudinal recording, recorded magnetization lies along the platter surface. Perpendicular magnetic recording (PMR) instead places the magnetization perpendicular to the surface. This describes the orientation of magnetic patterns in the medium—not a head that has simply been turned upright. Seagate describes the historical move from longitudinal to perpendicular recording as a way to support greater areal density. Seagate’s overview of hard drives and SSDs provides background on the recording technologies.
HAMR and the writeability tradeoff
As recording grains become smaller and more stable, they can better preserve dense data patterns, but they become harder for a writer to change reliably. HAMR addresses this tradeoff by briefly heating a tiny area of the platter so the writer can change a bit’s magnetic polarity. Seagate describes its implementation as using a laser diode attached to the recording head to provide that localized heat. Seagate’s HAMR explanation describes the approach.
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Seagate’s areal-density page says its Mozaic platform launched in January 2024 and identifies Mozaic as the company’s HAMR implementation. This is a dated Seagate platform milestone, not evidence that all HDDs use HAMR. Seagate’s areal-density explainer gives that account.
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How the approaches differ
| Recording approach | Magnetization orientation | Density and stability context | Localized heating to assist writing |
|---|---|---|---|
| Longitudinal recording | Along the platter surface. | Seagate describes the later move to perpendicular recording as supporting greater areal density; no specific density value is stated in the cited overview. | No localized heating is described in the cited sources. |
| PMR | Perpendicular to the platter surface. | Seagate describes PMR as supporting greater areal density than the preceding longitudinal approach; no specific density value is stated in the cited overview. | No localized heating is described in the cited sources. |
| HAMR | The cited Seagate material focuses on how heat assists writing rather than specifying a separate orientation here. | Seagate presents HAMR as addressing the difficulty of writing highly stable, small grains. | Yes. Seagate describes momentarily heating a tiny disk area with a laser diode attached to the recording head. |
What this means in practice
- The platter surface holds the data; the actuator positions the head over the track being accessed.
- The reader senses magnetic patterns, while the writer applies a field to change them.
- PMR describes magnetic orientation in the medium, while HAMR describes a way to assist writing highly stable bits.
- Head designs and recording methods vary. Seagate’s TMR reader and HAMR descriptions are examples of its technologies, not universal features of HDDs.
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