How a Hard Drive Works
In this article, we will cover the classic hard disk drive (HDD), as opposed to the SSD drive which functions like flash memory.
The hard drive is currently the most widely used storage medium.
It consists of a set of circular platters made largely of aluminum. These platters are stacked around the same vertical axis.
Each platter is coated with a magnetic layer that allows data to be recorded and read. This process is made possible by the rotation of the platters, which spin at thousands of revolutions per minute, along with the read heads that sweep across the surface of the platter, emitting or not emitting a magnetic field — we will come back to this later. The movement of the read head is managed by the disk controller.
The distance between the read head and the platter is on the order of one hundredth of a millimeter. The platters are so fragile that when a drive is opened outside a controlled environment, in most cases it becomes impossible to use the platters afterward. This can be caused by a speck of dust or a hair — a single contact with the platter is enough to permanently render it unusable.
A hard drive opened in Chronodisk’s cleanroom
Reading Data
A hard drive, like many other computer components, relies on binary logic.
The magnetic field generated by the read head can be positive or negative. This process is called polarization. The change in polarity as the head passes over is converted into a digital signal and interpreted by the computer as a 0 or a 1. The binary system, while simple to explain, remains complex in how it actually works.
Read/write heads are said to be “inductive”: they are able to generate a magnetic field that is positive or negative, allowing them to polarize a very small area of the disk surface. When this area is later read, the changes in polarity induce a current in the head, which is then converted by an analog-to-digital converter (ADC) into 0s and 1s that the computer can understand.
Data is always written to a platter from the outside in. The smallest memory area on a disk is called a cluster
Main Characteristics of a Hard Drive
- Storage capacity
- Platter density affects throughput — the higher this value, the higher the throughput.
- Access speed: the speed at which the read head moves between 2 blocks of data.
- Platter rotation speed: the higher the speed, the better the performance. However, the higher the speed, the noisier the drive
- Transfer rate is the speed at which data is written and read by the read head