This article has been just updated:
Modern storage devices look nothing like they did more than 50 years ago. The first hard drive in the world, the IBM 350, had fifty 24-inch platters, with a total capacity of 3.75 MB. Fast forward to today, and you can store the entire English Wikipedia uncompressed on a device the size of your thumb.
Storage devices have evolved at such an astonishing rate that it’s not easy to know which type is right for which application. In particular, many people wonder what’s the difference between Solid-State Drives (SSDs) and Hard Disk Drives (HDDs).
Technology
HDDs have been around since the 1950s, and it’s almost hard to believe how much they’ve changed over the years. Modern HDDs use magnetic storage to store and retrieve digital information using one or more rigid rapidly rotating disks, called platters, coated with magnetic material. Each platter has its own actuator arm with a read/write head, which moves to locate read and write data.
The platters typically spin at 5,400 or 7,200 or 10,000 revolutions per minute (RPM) and come in two main form-factors: 3.5-inch, which is intended for desktop computers and servers, and 2.5-inch, which is primarily for laptops and compact home theater PCs.
Modern HDDs are connected via Serial ATA (SATA), a computer bus interface introduced in 2000. SATA connects host bus adapters to mass storage devices and supports native hot swapping and faster data transfer through higher signaling rates, among other things. Some HDDs use Serial Attached SCSI (SAS) instead of SATA, but there’s actually little physical difference between the two.
SSDs started to appear in the 1990s, but their popularity as cutting-edge alternatives to HDDs is a relatively recent development. Instead of platters and actuators, they use integrated circuit assemblies as memory to store data persistently with no moving parts whatsoever. You can think of them as special USB drives because they use the same base technology: flash memory, also called NAND.
SSDs are designed to be drop-in replacements for traditional HDDs. Early SSDs were available in the 2.5-inch form factor, but the industry is now leaning toward the M.2 form factor, which is a more flexible physical specification that allows different module widths and lengths to be paired with the availability of more advanced interfacing features.
M.2 also supports the NVM Express (NVMe) specification for accessing non-volatile storage media attached via a PCI Express (PCIe) bus. NVMe allows SSD manufacturers to overcome the biggest limitation of SATA: bottlenecking.
“The interface provides an optimized command issue and completion path. It includes support for parallel operation by supporting up to 64K commands within a single I/O queue to the device” – explains NVM Express, which was formed as an industry association to define a new storage interface protocol, NVM Express.
Storage Capacity
Not too long ago, Samsung unveiled the world’s largest SSD, with whopping 30.72 TB of storage space. Shortly after, American computer data storage software and systems company Nimbus Data introduced an SSD with 100 TB of storage space. Clearly, SSDs can offer a lot of space, but it comes with a premium price.
The 1TB version of the Samsung 860 Evo, which is one of the most popular SATA SSDs on the market today retails for $199, although you can often find it for less than $130. The Western Digital Blue 1 TB hard drive, on the other hand, retails for $109.99 and is often on sale for less than $50. That’s a huge difference in price per GB, and it increases even more if you compare NVM Express SSDs with HDDs.
To save money and still enjoy the superior performance of SSDs, most users today buy a smaller SSDs as a boot drive and a larger HDDs for data. That way, your system will boot up in a few seconds, but you can still store hundreds of movies and thousands of songs on your computer.
Performance
The best NVM Express SSDs have a sustained throughput of around 3 GB/s, SATA SSDs reach around 550 MB/s, and HDDs manage only 200 MB/s. What do these numbers mean in practice? A massive difference in application load times and overall system responsiveness.
If you have an older laptop with a 5,400 RPM HDD and an Intel Core i5 processor, you can expect to wait around 15 seconds for the Chrome web browser to open. With an SSD, Chrome should open in just a second.
The difference can be even more noticeable in games. Playing Destiny 2, for example, on a traditional HDD gives you a lot of time to make yourself a cup of tea or coffee as you wait for the game to load. Even a budget SSD will barely give you enough time to open a bottle of your favorite soda before the game starts.
Reliability
If you’ve been paying attention, you should know that there are many moving parts inside traditional HDDs. The mechanical reality of all moving parts is that they wear out over time and may even fail catastrophically.
HDD manufacturers know this, which is why there’s an entire market with durable HDDs with increased lifespan and special safety features that are supposed to minimize shock damage, which typically occurs when the read-write head of the drive touches the magnetic platter. Unsurprisingly, extra-durable HDDs are more expensive than regular HDDs, which is why most consumers use regular HDDs instead. 
SSDs don’t have any moving parts, so they’re inherently more resistant to shock damage. However, they do wear out as well—just for different reasons. SSD memory cells have a finite life expectancy because they are subject to an effect called write amplification, an undesirable phenomenon associated with flash memory where the actual amount of information physically written to the storage media is a multiple of the logical amount intended to be written.
Despite the write amplification effect, SSDs have a significantly better MTBF (Mean Time Between Failures), and many SSD manufacturers include a 5-year or longer guarantee on their products.
Verdict
Today, there are not many reasons to buy a traditional hard disk drive. Solid-state drives have become very affordable, and their speed and reliability are on another level. The only good reason to buy a traditional hard disk drive is to use it to store large amounts of data for cheap. But even then, we recommend you use a solid-state drive as a boot drive for your operating system.
