{
  "slug": "drive-day",
  "term": "Drive-day",
  "aliases": [],
  "definition": "A drive-day is one drive observed in operation for one day, used as a unit of fleet exposure.",
  "note": "Exposure is additive: 100 drives observed for 10 days contribute 1,000 drive-days. Count only the time covered by the selected observation rule; a fleet's end-of-period headcount is not its exposure. Daily-presence records and SMART power-on hours measure different things and must not be silently substituted.",
  "status": "shipping",
  "advantages": [],
  "limitations": [],
  "related": [
    "afr",
    "smart",
    "right-censoring",
    "left-truncation"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "citations": [
    {
      "id": "bb-exposure",
      "title": "Backblaze: 10 Stories From 10 Years of Drive Stats Data, sections 3-4",
      "url": "https://www.backblaze.com/blog/10-stories-from-10-years-of-drive-stats-data/",
      "accessed": "2026-10-10"
    },
    {
      "id": "hesela-methods",
      "title": "Hesela: AFR is a rate, not a one-year failure probability",
      "url": "https://hesela.com/analyses/afr-rate-not-probability/",
      "accessed": "2026-10-10"
    },
    {
      "id": "hesela-age-audit",
      "title": "Hesela: Missing SMART ages and the limits of drive-age comparisons",
      "url": "https://hesela.com/analyses/backblaze-age-missing-data-audit/",
      "accessed": "2026-10-11"
    }
  ]
}