What’s Inside a Hard Disk?
2026-01-21
Although I have not conducted a thorough search, here is a concise overview of the components of a hard disk:
A hard drive consists of a spinning disk(s) made of aluminum and coated with a magnetic brown substance (an “oxide”). The read head is a transducer, a thin wire wrapped around a metal piece, sensitive enough to detect flux changes in the oxide on the disk platter as it rotates. It can detect data on the magnetic oxide much more slowly than we can access data from an array of memory cells etched on a silicon wafer. Read heads are suspended on arms that move them towards and away from the centre of the disk, controlled by a stepper motor (a simple motor that responds to digital pulses).
It is remarkable that we have used such cumbersome technology as hard drives for so long. We have developed extensive software (“file systems”) based on assumptions about these physical rotating disks. The software must synchronize with the rotation of the disk platters and determine which portion of data is under the read head.
To read a portion (a “sector”) of data from a disk, software must instruct the drive to move the read head arm to the appropriate position and must wait for the disk to rotate to the correct location.
The read heads move back and forth across the disk platters. The arms are fast, and the disk spins quickly, but CPU opcodes are much faster, so software must synchronize and determine the position of the read head arms at any given moment.
The software that synchronizes with the inward and outward movements of the read heads is called an elevator algorithm.
Floppy disks have a small hole punched in them, and use a photo sensor to detect when the hole passes by.
Floppy disks are similar to hard disks, except that the oxide is painted onto a floppy plastic, mylar disk instead of a rigid aluminum hard disk.
Since floppy mylar disks cannot spin as fast as rigid hard disks, the final access time for data is slower for floppy disks than for hard disks. However, floppy disk devices are significantly cheaper than hard disk devices.
Solid-state drives (SSDs) were once much more expensive to purchase than hard disks, but their prices have decreased sufficiently to make them economical for use in computers.
Despite these advancements, we continue to use “file systems” that were designed with spinning disks in mind.
See Also
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