EMZETT.Hard Drives
In short: Umbrella term for non-volatile mass storage in a computer — covers both classic magnetic HDDs and SSDs, even though “hard drive” originally only meant HDDs.
In more detail: Hard drives store data permanently, even without a power supply — unlike volatile RAM. They can be built in internally or connected externally via USB.
In Depth
Where the name comes from
The term “hard drive” historically comes from the rigid (“hard”) metal disk inside an HDD — as opposed to the flexible floppy disk, which used to exist in parallel as a removable, portable storage medium. With the rise of SSDs, which don’t contain a rotating disk at all any more but store data purely electronically in flash memory cells, the term was nevertheless kept colloquially and today simply means “the drive the operating system and files live on” — regardless of the underlying technology, even though, strictly speaking technically, only HDDs actually contain a “disk”.
Non-volatility as a shared property
All types of hard drives share the property of non-volatility: unlike RAM, they retain their data even after the power supply is switched off, which makes them indispensable for permanently storing the operating system, programs and personal files. This property is fundamentally different from RAM, which immediately loses its content when switched off — hard drives are therefore the only place where data survives a restart or power outage.
Three purchasing criteria
For practical selection, three factors mainly matter together: capacity (how much data fits, from a few hundred gigabytes to several tens of terabytes for current HDDs), speed (how fast data is read/written, decisive for the perceived system speed — the difference between HDD and SSD is often the most noticeable performance jump you can give an older PC at all), and price per gigabyte — SSDs win clearly on speed, HDDs remain cheaper on pure price per gigabyte for very large capacities, which is why many systems today use a combination of both (fast SSD for the operating system and frequently used programs, large HDD for archive data).
Reliability and lifespan
Both technologies are subject to different failure mechanisms: HDDs can fail through mechanical wear of the rotating parts or shocks, SSDs through the limited write cycles of the flash memory cells (in practice, not a relevant problem under normal everyday use, since modern SSDs are designed for very many write cycles). Regardless of technology: every hard drive can fail, which is why regular backups on a separate medium remain indispensable.
See also: HDDs, SSDs, Internal Hard Drives, External Hard Drives, RAM