Why Western Digital and Seagate are seeing big stock drops today

The selloff in Western Digital and Seagate reflects a straightforward shift in supply expectations. Investors are reacting to the possibility that Toshiba will expand output of nearline hard drives—the high-capacity units used in data centers to store AI training data. Until now, demand for that storage has outpaced supply, giving incumbent manufacturers unusual leverage over pricing.

That pricing power is the crux of the story. When supply is tight, drive makers can hold the line on contract prices and protect margins even as volumes rise. The prospect of additional capacity from a major competitor introduces the opposite scenario: more product chasing the same demand, which tends to pressure prices over time.

What to watch is whether Toshiba's plans translate into actual shipments, and on what timeline. Announced capacity additions often take quarters to materialize, and AI-related storage demand remains difficult to forecast. For now, the market is repricing the durability of today's favorable supply-demand balance rather than reacting to confirmed oversupply.