How Memory and Storage Power AI: HBM, DRAM, NAND, HDD

Every AI answer starts with data moving from storage to a processor. A modern GPU can do math far faster than memory can feed it, so the AI build-out is as much a memory and storage story as a chip story. Five names come up most often: Micron (MU), SK hynix, Sandisk (SNDK), Western Digital (WDC) and Seagate (STX). They sell different products at different layers of the same stack. This guide explains each layer, what each company makes, and how the pieces fit together.

The memory and storage pyramid

No single technology is fast, large and cheap at once. Data centers stack several, with the fastest and most expensive closest to the processor and the cheapest and slowest furthest away.

DRAM, NAND and hard drives in plain English

DRAM is the computer's working memory. It is very fast but loses its contents when power is cut, so it holds only what is being used right now. HBM, or high bandwidth memory, is DRAM re-engineered for AI: several DRAM chips are stacked vertically, linked by thousands of tiny vertical connections, and placed on the same package as the GPU. That short, wide path delivers far more data per second than ordinary memory slots. Because HBM needs more silicon per bit and specialized packaging, adding it can also tighten the supply of conventional DRAM, which is one reason memory prices have moved so sharply.

NAND flash keeps data without power and sits in SSDs. It is slower than DRAM but far cheaper per bit, and makers keep raising capacity by stacking more layers. SK hynix, for example, said 321-layer products became the largest share of its NAND production in the second quarter. Hard drives store data magnetically on spinning platters. They are the slowest tier, but they remain the lowest-cost option for bulk capacity, which is why cloud providers still buy them by the exabyte.

Why AI inference raised the stakes

When a model answers a long prompt or runs an AI agent, it keeps a running record of the conversation, called the KV cache, so it does not recompute everything each time. The longer the context, the bigger the cache. It starts in HBM, spills to server DRAM when HBM fills up, and can then spill to fast SSDs. Micron said its PCIe Gen 5 and Gen 6 data center SSDs are shipping to leading customers for KV-cache applications. Sandisk and SK hynix have also published a first specification for high bandwidth flash (HBF), a NAND-based tier meant to sit between DRAM and SSDs for inference at scale, as a complement to HBM rather than a replacement.

The five companies at a glance

Micron: DRAM, HBM and flash under one roof

Micron, based in the United States, makes both DRAM and NAND, so it touches three of the four tiers: HBM, server memory and SSDs. In its fiscal fourth quarter, revenue was $54.23 billion, up from $11.32 billion a year earlier, or about 4.8 times, by our calculation. GAAP gross margin was 86.8%. Its two data center-focused units, Core Data Center ($18.00 billion) and Cloud Memory ($16.28 billion), together made up about 63% of quarterly revenue, by our calculation. Micron said revenue from its server LPDDR SOCAMM modules more than doubled sequentially, that it has begun sampling a 512GB DDR5 server module, and it guided fiscal first-quarter revenue to $61.5 billion, plus or minus $1.5 billion. Net capital spending was $27.37 billion for the full fiscal year.

SK hynix: the HBM specialist

SK hynix is based in South Korea and also sold ADRs in the United States in July 2026. It is best known as a leading supplier of HBM to AI accelerators. In 2Q26 it reported revenue of ₩79.32 trillion, up 257% from a year earlier, and an operating profit of ₩60.54 trillion, a 76% margin. The company said HBM4 mass shipments began in the quarter, with production ramping in the second half, and that HBM4E samples shipped in the first half. It also cited rising DRAM prices, growing sales of SOCAMM2 modules and enterprise SSDs, and long-term supply agreements with about 10 customers. It said the ability to deliver requested volumes on time has become a core competitive strength.

Sandisk: pure-play NAND flash

Sandisk was separated from Western Digital in February 2025 and now focuses on flash storage, which it makes with partner Kioxia. Its fiscal fourth-quarter revenue was $8.97 billion, up 51% sequentially and 372% from a year earlier, with a gross margin of 84.6%. Sandisk said about two-thirds of the sequential growth came from higher pricing and one-third from volume. Data center revenue was $2.98 billion, about 33% of the quarter's revenue, by our calculation, and fiscal-year data center revenue reached $5.15 billion, up 437%. Edge devices such as PCs, phones and cars were the largest end market at $5.43 billion. The board approved an additional $14 billion buyback, and Sandisk guided fiscal first-quarter revenue to $10.30 billion–$10.80 billion.

Western Digital and Seagate: where the bulk data lives

Since the Sandisk separation, Western Digital sells hard drives. Its fiscal fourth-quarter revenue was $3.75 billion, up 44% from a year earlier, with a GAAP gross margin of 54.1%, and it guided the next quarter to $4.1 billion, plus or minus $100 million. Seagate reported $3.63 billion, up about 48% from $2.44 billion a year earlier, by our calculation, with a non-GAAP gross margin of 52.7% and fiscal-year revenue of $12.20 billion, up 34%. It guided its next quarter to $4.1 billion, plus or minus $100 million.

Both are chasing the same goal: more terabytes per drive. Seagate's Mozaic platform uses heat-assisted magnetic recording (HAMR) to pack data more tightly. According to a November 2025 Zacks analysis, Mozaic 3+ drives hold up to 36 terabytes, and Seagate has targeted 5 terabytes per disk in early 2028. Seagate CEO Dave Mosley said, "As AI accelerates data generation and its value, we see durable long-term demand for mass capacity storage."

How a piece of data travels through the stack

Each step trades capacity for speed. A shortage or breakthrough in any one tier can ripple through the rest, which is why investors track all of them.

Why memory stocks swing

Memory has historically been a commodity business, where prices rise and fall with supply and demand and profits swing with them. In the latest quarters the swings have been unusually large, with Sandisk crediting most of its sequential growth to price. Makers are trying to smooth the cycle with multi-year supply agreements, such as SK hynix's agreements with about 10 customers, and with spending discipline. Micron's annual net capex of $27.37 billion and SK hynix's phased plans for new fabs and packaging show how much capacity is being added. Whether that discipline holds if demand slows is the central question for the group.

Key terms

Sources: Micron fiscal Q4 2026 release (SEC filing); Sandisk fiscal Q4 2026 release; Western Digital fiscal Q4 2026 release (SEC filing); Seagate fiscal Q4 2026 release (SEC filing); SK hynix 2Q26 results and newsroom; Zacks analysis, Nov. 2025. This article is for education and is not a recommendation to buy or sell any security.