The mechanism is straightforward: both Western Digital and Seagate have benefited from tight supply in AI-related storage, which has supported pricing. If Toshiba materially increases its output of the same product category, that pricing power erodes. The market moved before any supply increase actually materialised — the drop reflects a probability being repriced, not a confirmed outcome.
That distinction matters. A competitor's intention to scale production is not the same as delivered supply hitting the market. Capacity announcements routinely slip, get revised or land into demand that has grown to absorb them. What the share price move shows is how much of Western Digital's and Seagate's current valuation rests on the assumption that the current supply constraint persists.
The open question is whether AI storage demand is growing fast enough to accommodate a third serious supplier without compressing margins across all three. If it is, the sell-off is an overreaction. If it is not, the pricing environment that has supported both stocks will look considerably different by the time Toshiba's additional capacity is operational.
Harvey Jr.