{
  "slug": "hdd",
  "term": "Hard disk drive (HDD)",
  "aliases": [
    "hard drive",
    "magnetic disk"
  ],
  "definition": "A non-volatile storage device that records data on spinning platters with a moving read/write head.",
  "note": "An HDD stores bits as magnetic orientations on spinning platters; one or more heads on an actuator arm seek radially while rotational latency waits for the sector to pass under the head. Capacity scales with platter count, areal density (PMR/CMR/SMR plus assisted recording such as HAMR/MAMR), and packaging (air vs helium). Interfaces today are predominantly SATA or SAS for block access; performance is bounded by mechanics (IOPS and latency) rather than flash controller parallelism. HDDs remain the common economics choice for warm/cold bulk and nearline tiers where dollars-per-TB and watts-per-TB matter more than microsecond latency. Form factors in volume are mainly 3.5-inch enterprise/nearline and 2.5-inch mobile/legacy.",
  "status": "shipping",
  "advantages": [],
  "limitations": [],
  "related": [
    "cmr",
    "smr",
    "pmr",
    "nearline-hdd",
    "sata",
    "sas"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": {
    "date": "1956",
    "citationIds": [
      "c1"
    ]
  },
  "lastSeen": null,
  "citations": [
    {
      "id": "c1",
      "title": "IBM 350 disk storage unit",
      "url": "https://www.ibm.com/history/ibm-350-disk-storage-unit",
      "accessed": "2026-03-26"
    },
    {
      "id": "n2",
      "title": "Seagate — CMR and SMR hard drive list (mass-capacity context)",
      "url": "https://www.seagate.com/products/cmr-smr-list/",
      "accessed": "2026-10-07"
    },
    {
      "id": "n3",
      "title": "Western Digital — Helium hard drives explained (mechanics packaging)",
      "url": "https://blog.westerndigital.com/helium-hard-drives-explained/",
      "accessed": "2026-10-07"
    },
    {
      "id": "n4",
      "title": "IDEMA — HDD technology resources (industry association)",
      "url": "https://idema.org/",
      "accessed": "2026-10-07"
    }
  ]
}