How does a hard drive actually work?
By writing magnetic patterns onto spinning platters and reading them back with a head that flies microscopically close to the surface without ever touching it — a mechanical arrangement of remarkable precision that is now being displaced from everything except bulk storage.
The components: one or more rigid platters coated with a magnetic layer, spinning at typically 5,400 or 7,200 revolutions per minute; read/write heads on an actuator arm; and a controller managing the whole thing.
How data is stored. The magnetic coating is divided into billions of tiny regions, each magnetised in one of two directions to represent a bit. The write head produces a field that flips them; the read head detects the transitions.
The flying height is the extraordinary part. The head rides on a cushion of air generated by the spinning platter, at a separation measured in nanometres — far smaller than a smoke particle. If it touches, the result is a head crash, physically destroying the surface. This is why a drive dropped while running is far more likely to fail than one dropped while off, and why drives are sealed.
Why they are slow relative to solid-state. Any read requires the head to move to the right track — seek time — and then wait for the data to rotate underneath it — rotational latency. These are mechanical delays measured in milliseconds, while flash memory responds in microseconds. Random access is where the gap is enormous; sequential transfer is much closer.
Why they still exist. Cost per terabyte remains substantially lower, which is why data centres, backups and archives still use them in vast numbers.
How they fail: mechanically, with clicking or grinding noises indicating head or motor problems; through gradual accumulation of bad sectors; and through controller electronics failure. Failure is frequently gradual and detectable through self-monitoring attributes, particularly reallocated sector counts — which are worth checking rather than waiting.
Practical points: a clicking drive should be powered off immediately, since continued operation worsens physical damage; and recovery from a mechanically failed drive requires a clean room and is expensive, which is an argument for backups rather than heroics.