Nearline (high-capacity enterprise) HDD is a density-led, supply-disciplined ~3-player oligopoly chokepoint, allocated through 2027 — mechanically DIFFERENT from the memory-supercycle chip cycle and NOT a supply-flood-driven cycle roll. (1) Density-led margins, not cycle-led: Seagate (HAMR/Mozaic) and WDC (ePMR/UltraSMR) grow exabytes via areal density on flat bill-of-materials, capex pinned 4-6% of revenue — margin expansion from density, not a self-correcting price spike. (2) Disciplined oligopoly: STX ~40% / WDC ~42% / Toshiba ~18% (>95% combined); the two majors EXPLICITLY refuse to add unit capacity, so supply does not flood the way memory does — no 2027 capacity cliff breaks it from the supply side. CAVEAT: Toshiba's discipline is inferred, not quoted — the supply-side soft spot. (3) Captive moats: heads, recording media, and the HAMR plasmonic writer are made in-house at STX/WDC, so the drive makers themselves are the purest plays. Demand corroborated by primary sources (Seagate Q3 FY26: nearline 'almost fully allocated through calendar 2027'; WDC: lead times ~a year, 'not adding unit capacity'; TrendForce >52-week lead times). Set-up 2026-06-03: STX/WDC blown off (RSI ~80,…