Hardware
HDD
Also known as: Hard Disk Drive, Hard Drive, Spinning Drive, Mechanical Drive
Stores data on spinning magnetic platters read by a mechanical arm. Highest capacity per dollar of any storage medium, but significantly slower than SSDs and susceptible to physical shock.
HDDs read and write data by moving a physical read/write head over spinning magnetic platters. This mechanical nature is both their limitation and their value: the physics of spinning media constrains random I/O performance, but the same physics allows high areal density and large capacities at competitive cost per terabyte.
Where HDDs still make sense
High-capacity storage at low cost per TB — for backup targets, archival storage, and large media storage where access speed isn't critical, HDDs offer 4-20TB+ drives at a fraction of the SSD cost per TB.
NAS arrays for bulk storage — Synology and QNAP NAS units running RAID arrays for file sharing and backup are a classic HDD use case. Sequential read performance is adequate; the bottleneck is usually the network, not the drives.
Secondary tiers in tiered storage — an HDD tier holds infrequently accessed data behind an SSD cache or tier that handles hot data.
Where HDDs don't belong
OS and application drives — the random I/O latency of HDDs (typically 5-15ms per operation vs <0.1ms for NVMe SSDs) makes for a noticeably worse experience. Anything running an operating system or database should be on SSD.
High-IOPS workloads — database transaction logs, virtual machine storage, anything that generates significant random I/O will be severely bottlenecked by HDD performance.
Failure characteristics
HDDs have moving parts and fail mechanically. Enterprise HDDs are rated for MTBF (Mean Time Between Failures) of 1-2 million hours, but drives in dense arrays fail regularly in practice. RAID provides redundancy; regular backups are still non-negotiable. SSDs fail differently (and usually more gracefully) but also fail.