Solid state drives (SSDs) have become increasingly popular in recent years as a replacement for traditional hard disk drives (HDDs). SSDs offer much faster read and write speeds, better durability, and lower power consumption. However, SSDs have historically been produced in smaller form factors like 2.5 inches rather than the larger 3.5 inch size used for most HDDs. This has left some wondering why no 3.5 inch SSDs exist.
To understand why 3.5 inch SSDs are not common, it helps to first understand what makes SSDs different from HDDs. SSDs store data on flash memory chips rather than magnetic platters like in HDDs. This allows SSDs to access data much faster, with typical read/write speeds over 500MB/s compared to 80-160MB/s for HDDs. However, flash memory chips are more expensive per gigabyte than magnetic storage, so SSDs have historically offered less overall storage capacity than similarly sized HDDs. The smaller 2.5 inch SSD form factor helped keep costs down compared to 3.5 inch HDDs. over time, flash memory prices have decreased and SSD storage capacities have increased, making them suitable replacements for HDDs in many applications. But the 2.5 inch form factor has remained the de facto standard for SSDs in both consumer and enterprise storage solutions.
Cost and Capacity
The main reason 3.5 inch SSDs are uncommon comes down to cost and storage capacity. Producing SSDs in the larger 3.5 inch size would require substantially more raw flash memory than a 2.5 inch SSD with the same capacity. For example, a 2TB 2.5 inch SATA SSD might use 16 or more individual flash memory packages, while a 3.5 inch SSD would need 36 or more flash packages to offer the same 2TB of poo storage. With current flash memory pricing, this makes the 3.5 inch SSD significantly more expensive to produce. Considering most consumer and business buyers are motivated by the superior performance per dollar of SSDs, increased costs for 3.5 inch variants limits their appeal. As manufacturing output scales up, this cost disparity may shrink over time. But for now, most SSD makers are maximizing profits by focusing on 2.5 inch models that more closely align with HDD price points.
Heat also plays a role in the lack of 3.5 inch SSD adoption. Flash memory chips generate more heat than mechanical hard drive platters and actuators. This requires SSDs to incorporate thermal management solutions like heat spreaders and throttling. Building an SSD in a larger 3.5 inch enclosure would increase the difficulty of dissipating heat from the packed flash memory chips. Most 2.5 inch SSDs rely on the metal frame and enclosure for structural heat spreading, but pushing up capacity in a larger 3.5 inch variant would likely require more active cooling. The additional engineering and production costs of managing thermal output have not yet made 3.5 inch SSDs enticing for manufacturers relative to customer demand.
Drive Bays and Backplane Support
The widespread availability of HDD bays and backplanes for 3.5 inch drives has also slowed adoption of 3.5 inch SSDs. Most modern desktop PCs, servers, and external enclosures are designed to accommodate the well-established 3.5 inch HDD form factor. Transitioning these existing products to support a new 3.5 inch SSD standard would require overcoming engineering hurdles and inventory challenges. SSD makers have benefited from focusing on backwards-compatible 2.5 inch drive designs that can leverage the existing ecosystem of drive bays, trays, and backplanes meant for HDDs. Until 3.5 inch SSD demand increases drastically, these legacy compatibility challenges favor continued 2.5 inch SSD production.
NAND Flash Wafers
At the component level, the NAND flash memory wafers used to build SSDs also influence form factors. Most planar and 3D NAND flash is fabricated on 300mm wafers. These wafers are then cut into smaller dies that are packaged into NAND flash components. Producing a 3.5 inch SSD would require either larger wafer sizes or more NAND flash packages per drive. However, 300mm wafers remain dominant due to optimized manufacturing processes and equipment. And higher package counts would raise costs. Instead, SSD fabs have focused on scaling down die sizes and increasing density within existing 300mm wafer constraints. Unless wafer sizes increase, the transition to 3.5 inch SSDs would require major changes in fabrication and NAND flash memory packaging that suppliers have so far been unwilling to undertake.
Enterprise SSD Use Cases
Although uncommon in the consumer space, there are some use cases where 3.5 inch SSDs deliver significant value for enterprise and data center customers. Hyperscale cloud providers like Amazon, Google, and Facebook have deployed specialized rackmount servers using 3.5 inch SSDs. These custom systems are designed for density by packing four or more high capacity SSDs into a 1U server. Facebook’s Bryce Canyon storage servers, for example, leverage twelve 3.5 inch SSD slots in a 1U form factor. For large-scale data centers where space, power, and cooling are limited resources, the ability to maximize SSD capacity per server can offset the higher costs of 3.5 inch variants.
M.2 and U.2 Form Factors
Although 3.5 inch SSDs are rare in the consumer space, smaller form factors like M.2 and U.2 have gained adoption for enterprise and data center use. The M.2 form factor uses SSDs on a removable card just 22mm wide and 30/42/60/80mm long. These miniature SSDs are designed to plug into an M.2 slot on a server or storage system motherboard. The U.2 (formerly SFF-8639) form factor uses SSDs in a 2.5 inch drive form factor but with a thinner 15mm height. Both M.2 and U.2 deliver high performance and density for space-constrained server storage applications. Their success indicates the market prefers evolving smaller SSD form factors to alternatives like 3.5 inches for enterprise environments.
Another key technology shift further reducing the need for 3.5 inch SSDs is the rise of NVMe (Non-Volatile Memory Express). NVMe is a host controller interface and protocol designed to replace SATA and SAS for interfacing with SSDs. By providing higher queue depths, lower latency, and better parallelism, NVMe extracts maximum performance from high-speed NAND flash storage. NVMe works over the PCIe bus rather than legacy storage buses like SATA. This allows NVMe SSDs in compact M.2 and U.2 form factors to offer vastly better performance than 2.5 or 3.5 inch SATA SSDs. The industry momentum behind NVMe helps explain the focus on smaller SSD form factors rather than 3.5 inch models.
PCIe Bus Bandwidth
Higher PCIe bus bandwidth has also enabled smaller yet higher performance SSDs. M.2 NVMe SSDs are connected via PCIe 3.0 or PCIe 4.0 lanes, which offer substantially more bandwidth than SATA. A x4 PCIe 4.0 interface provides up to 7.88GB/s of bandwidth, while even SATA III tops out at 600MB/s. This allows M.2 SSDs to maximize the capabilities of high speed NAND flash. Larger 3.5 inch SSDs would be bottlenecked by SATA bandwidth limits, negating their performance potential. The industry has favored adopting faster interfaces like NVMe and PCIe 4.0 for SSDs rather than moving to larger form factors.
In summary, 3.5 inch SSDs have yet to hit the mainstream consumer market due to a combination of cost, engineering, and industry momentum challenges. Creating cost effective and high capacity 3.5 inch SSDs would require major investments and process changes by flash memory and SSD manufacturers. With existing 2.5 inch SSDs able to effectively replace 3.5 inch HDDs in most applications, there has been little motivation to overcome these hurdles so far. Looking forward, smaller form factors like M.2 and U.2 seem poised to dominate high performance storage over potential 3.5 inch SSDs. But niche use cases like hyperscale data centers may eventually justify bringing 3.5 inch SSDs to market if their substantial capacity needs overcome the higher costs.