Solid state drives (SSDs) and hard disk drives (HDDs) are two different types of computer storage devices. SSDs store data on integrated circuits, while HDDs store data on magnetic disks. A key difference between SSDs and HDDs is the capacity or amount of data they can hold. SSDs typically have lower capacities compared to HDDs. This is because SSDs are more expensive per gigabyte compared to HDDs. However, SSDs are faster, more durable, energy efficient, and compact. This guide will examine SSD and HDD capacities in detail.
How HDD Capacity Works
HDDs store data on spinning magnetic platters. The platters are made of glass, ceramic, or aluminum and are coated with a magnetic material. As the platters spin, a read/write head floats above the surface and magnetizes or detects the magnetization of the platter in specific locations to encode the data (https://forums.tomshardware.com/threads/asrock-ep2c602-2l-d16-hdd-capacity-limits.3696815).
The capacity of an HDD is determined by the number of platters it contains. More platters equals more surfaces for storing data, which results in higher total capacity. For example, most desktop HDDs have between 1-5 platters, while high capacity datacenter HDDs can have up to 10 platters (https://www.fujitsu.com/lu/about/resources/news/press-releases/2012/Fujitsu-Helps-Businesses-Overcome-HDD-Capacity-Limits-with.html).
In addition, areal density, which is the amount of data that can be stored on a given surface area of the platter, has steadily increased over time. Higher areal density means more data can be packed onto each platter surface. Areal density has grown exponentially, approximately doubling every two years, enabling massive HDD capacity growth over decades (http://dma.vgtu.lt/KTA/KTA5_EN.pdf).
HDD Capacity Evolution
Hard disk drive (HDD) capacities have grown rapidly over the decades, starting from just a few megabytes initially. This massive growth in HDD capacities has been driven by continual improvements in areal density, which is the amount of data that can be stored on a given surface area of an HDD platter. Back in the 1980s, HDDs typically had capacities of 10-20MB. By the late 1990s, HDD capacities had reached into the gigabytes, and by the early 2000s they were reaching over 100GB.
Through steady areal density improvements decade after decade, enabled by new technologies like perpendicular recording and shingled magnetic recording, HDD capacities have steadily grown larger. In recent years, capacities of consumer HDDs have gone from 4TB to 10TB to 16TB. High capacity enterprise hard drives are even larger, with the world’s largest HDDs topping out around 16TB currently. While growth is slowing, advancements in recording technologies will likely push capacities even higher in the future.
How SSD Capacity Works
SSDs store data in NAND flash memory chips. The more NAND chips an SSD has, the more total capacity it can offer. For example, a 240GB SSD may contain 16 NAND flash chips of 16GB each for a total of 256GB of raw capacity. After reserving some capacity for redundancy and over-provisioning, the usable capacity is around 240GB.
NAND flash density has increased over time, allowing each individual chip to store more data. Early SSDs used single-level cell (SLC) NAND flash, which stored 1 bit per memory cell. Newer SSDs use multi-level cell (MLC) or triple-level cell (TLC) NAND flash, which can store 2 or 3 bits per cell respectively. This increased density allows SSD manufacturers to offer greater capacities without necessarily adding more chips.
For example, according to ExtremeTech, early SSDs in 2009 had capacities of 64-256GB. But by 2019, 2TB SSDs had become common, with densities up to 1Tb per NAND flash chip. This exponential growth in density has enabled the rise in SSD capacity over a relatively short period of time.1
SSD Capacity Growth
SSDs started with much lower capacities compared to HDDs when first introduced. The first SSDs in 1991 had a capacity of only 20MB (Reference 1). In comparison, HDD capacities at that time were already in the range of tens of gigabytes.
However, SSD capacities have grown exponentially over the years thanks to rapid advances in NAND flash density. By 2009, SSD capacities reached 256GB, and by 2011 they reached 512GB (Reference 2). This allowed SSDs to become a viable replacement for HDDs in more use cases.
SSD capacity growth has accelerated in the 2010s and 2020s. Today, consumer SSDs are available in capacities up to 8TB, while enterprise SSDs can reach 64TB (Reference 3). Experts predict SSD capacities will continue to grow rapidly, with 100TB+ drives expected by 2025.
References:
1. https://www.thestack.technology/the-evolution-of-storage-alex-mcmullan-pure-storage/
2. https://insights.samsung.com/2021/12/22/the-evolution-of-solid-state-drives-ssds/
3. https://www.linkedin.com/pulse/solid-states-drives-ssd-market-size-growth-forecast-from/
Comparing HDD and SSD Capacities
While HDDs traditionally had much higher maximum capacities than SSDs, there is now significant overlap in the ranges. As recently as 2018, consumer HDD capacities topped out at 10 TB, while 2 TB was the maximum for consumer SSDs (Source). However, SSD capacities have rapidly expanded, with consumer SSDs now reaching 8 TB and HDDs hitting 20 TB (Source). This convergence in capacities makes SSDs viable for many consumer use cases where large HDDs were previously required.
For the average consumer use like laptops, gaming PCs, and home media storage, SSDs in the 500 GB to 2 TB range have overtaken HDDs in popularity due to their speed, durability, form factor, and energy efficiency. HDDs retain dominance in the highest capacities above 8 TB, where the lower cost per gigabyte makes them preferable for bulk data storage (Source). However, SSDs now offer sufficient capacity for most common home and business uses.
Factors Affecting HDD Capacity
There are several key factors that determine the maximum capacity of a hard disk drive (HDD):
Number of platters – HDDs consist of round platters coated with magnetic material that store data. More platters allow for more data storage capacity. High capacity HDDs have up to 8 or more platters.1
Areal density – This refers to the amount of data that can be stored on a given surface area of the platter. Higher areal density, measured in gigabits per square inch (Gb/in2), allows for greater storage capacity per platter. Areal density has steadily increased over decades, enabling higher capacity HDDs.2
Reliability and error correction overhead – More advanced error checking and correction requires reserving space for redundancy and metadata. This overhead can reach 2-3%, reducing user available capacity.3
Factors Affecting SSD Capacity
The capacity of an SSD is primarily determined by three factors:
Number of NAND chips: SSDs contain multiple NAND flash memory chips. Each chip contributes to the total capacity. More chips means higher potential capacity.https://osgamers.com/faq/why-only-931gb-on-a-1tb-ssd
NAND lithography process: This refers to the size of the smallest features on the NAND chips. Smaller lithography allows for denser data storage. Moving from 34nm to 15nm doubled SSD density.https://www.reddit.com/r/hardware/comments/1l438u/ssds_are_here_to_stay_as_new_memory_technology_is/
Over-provisioning needs: SSDs reserve extra capacity to improve performance and endurance. Typically ~7% is reserved by the manufacturer. More over-provisioning means less usable capacity.https://osgamers.com/faq/what-to-do-if-ssd-is-too-small
Capacity Needs for Different Uses
Capacity requirements vary widely depending on the intended use case. For consumers using SSDs or HDDs in laptops or desktop PCs, capacity needs typically range from around 500GB to 2TB. This provides enough space for the operating system, programs, files, photos, videos, music, and other data for most home users. According to PCMag, 500GB is considered a “base” HDD capacity for premium laptops these days.
For businesses and enterprises, capacity needs are much higher, especially when using HDDs for storage servers, SANs, and network-attached storage. Business HDD capacities can range from 4TB up to 10TB or even higher for applications like big data analytics, databases, file servers, backups, and archives. SSD capacities for business uses also continue to grow into the multi-terabyte range. But HDDs still offer a significant cost advantage for high capacity bulk storage needs.
As SuperUser notes, it’s also recommended to leave 15-20% of HDD capacity free, and 10% or more free space on SSDs, to allow for optimal performance as the drives fill up over time.
Future Outlook
SSD capacities will continue expanding thanks to advancements in NAND density. Multi-layer 3D NAND and improvements in 3D layer stacking are already allowing SSD capacities up to 128TB and higher. In the coming years, NAND density will enable SSDs to reach capacities of 10s of TBs for the consumer market and up to 1PB for enterprise data centers.
HDDs will also see gradual capacity increases, to over 50TB by 2025. Technologies like shingled magnetic recording will help push HDD capacities up, though more incremental advancements in areal density will be the primary driver of capacity growth. While SSDs will continue growing exponentially, HDDs will retain their advantage for extremely high capacity bulk storage needs, into the 10s of PBs.
