Shares of hard disk drive giants Seagate Technology (STX) and Western Digital (WDC) have skyrocketed, drawing attention to a storage sector that was largely left for dead just a few years ago. Both companies have posted remarkable revenue recoveries: STX reported ~$9.1B in revenue for FY2025 (ending June 2025), up roughly 39% YoY, while WDC posted ~$9.5B, up a staggering ~51% YoY. WDC carries stronger gross margins at 38.8% versus STX's implied thinner margins, though STX delivered $6.77 diluted EPS versus WDC's $5.12 — suggesting better earnings efficiency at STX relative to revenue.

The backdrop is a massive AI-driven boom in nearline HDD demand, as hyperscalers and cloud providers scale out storage capacity to support model training and inference workloads. Both companies sit in a near-duopoly with Toshiba HDD, giving them pricing power in an upcycle. WDC also carries flash memory exposure through its NAND segment, adding another vector of cyclical recovery.

The bull case rests on continued hyperscaler capex expansion and the secular trend toward higher-capacity HDDs (e.g., HAMR/MAMR technology), which could sustain pricing and margin expansion well into 2026. The bear case is that HDD cycles have historically been violent — inventory builds and customer digestion periods can snap revenue growth quickly — and both stocks may already reflect the bulk of the upcycle in their current valuations after a large move up.

Watchers should track hyperscaler capex commentary (AWS, Azure, Google) for any signs of nearline HDD order moderation. WDC's ongoing NAND segment and potential flash business separation adds a wildcard. With earnings likely reflecting peak-cycle dynamics, the key debate is duration of the upcycle, not its existence.