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If you bought a laptop or desktop PC anytime in the last few years, you almost certainly have a solid-state drive (SSD) as your computer's primary boot drive. You can rest assured that you're using the best storage technology available today. However, there's an alternative for your secondary-storage needs: the long-familiar hard disk drive (HDD). In general, an HDD will be much slower than an SSD because it uses a vastly different, older storage technology—a spinning platter instead of memory cells. As a result, HDDs have nowadays been reduced to a secondary role in the consumer storage market.
We can think of a few compelling reasons, though, that you'd still want an HDD, especially when buying an external drive for data backups or in a desktop PC with more than one drive. If you're considering buying or upgrading one of these, you'll need to understand the differences between SSDs and HDDs. We'll walk you through the advantages and disadvantages of each so you can decide which one to choose.
The Basics: HDD and SSD Explained
The traditional spinning HDD is the basic non-volatile storage on a computer. (Unlike data stored in RAM, information on it doesn't "go away" when you turn off the system.) A hard drive is essentially a metal platter or stack of platters with a magnetic coating that stores your data. A read/write head on an arm (or a parallel-mounted set of them) accesses the data while the platters are spinning.
An SSD performs the same basic function as a hard drive, but data is instead stored on interconnected flash-memory chips that retain the data even when no power flows through them. These flash chips (often dubbed "NAND") are different than those used in USB thumb drives, and are typically faster and more reliable. SSDs are consequently more expensive than USB thumb drives of the same capacities. (See our deep-dive guide to SSD jargon.)
Like thumb drives, though, SSDs are often much smaller than HDDs and, therefore, offer manufacturers more flexibility in designing a PC. While some can be installed in traditional 2.5-inch or 3.5-inch hard drive bays, other models can be installed in a PCI Express expansion slot or even be mounted directly on the motherboard, a configuration that's now common in laptops and desktops. (In most cases, these board-mounted SSDs use a form factor known as M.2. See our picks for the best M.2 SSDs and get much more info on these multifaceted types of SSDs.)
Note: We'll be talking primarily about internal drives in this story, but almost everything also applies to external drives. External drives come in both large desktop and compact portable form factors.
A History of HDDs and SSDs
HDD technology is relatively ancient (in terms of computer history, anyway). The IBM 650 RAMAC hard drive from 1956 used a whopping total of 50 24-inch-wide platters to hold a mere 3.75MB of storage space. That's roughly the size of a single music track you might stream today, stored in the physical space that could hold two commercial refrigerators.
The PC hard drive form factor was standardized at 5.25 inches in the early 1980s, with 3.5-inch desktop-class and 2.5-inch notebook-class drives coming soon after that. The internal cable interface has changed over the years from serial to IDE (now frequently called Parallel ATA, or PATA) to SCSI to Serial ATA (SATA). But each essentially does the same thing: connect the hard drive to the PC's motherboard so your data can be shuttled to and fro. The 2.5- and 3.5-inch drive sizes are what survive today in modern internal HDDs, and both use SATA interfaces in almost all cases (at least on consumer computers). As for capacities, HDDs have grown from multiple megabytes to multiple terabytes, more than a million-fold increase, while retaining their same 2.5-inch and 3.5-inch physical sizes.
The solid-state drive has a much shorter history, though its roots do reach several decades into the past. Technologies like bubble memory were briefly popular in the 1970s and 1980s. Those bubbles have long since burst, but current flash memory is the logical extension of the same idea, as it doesn't require constant power to retain the data you store on it. The first primary drives we know as SSDs started appearing during the rise of so-called "netbooks" in the late 2000s. Capacities were tiny by today's standards. In 2007, the OLPC XO-1 used a 1GB SSD, while the Asus Eee PC 700 series used a 2GB SSD as primary storage. The SSD chips on these laptops were permanently soldered to the motherboard.
As netbooks and other ultraportable laptops became more capable, SSD capacities increased and eventually standardized on the 2.5-inch notebook form factor. This way, you could pop a 2.5-inch hard drive out of your laptop or desktop and replace it easily with an SSD, and manufacturers could design around just one kind of drive bay.
In time, other, more compact SSD form factors emerged, like the mSATA Mini PCIe SSD card and the aforementioned M.2 SSD format (which comes in SATA and PCI Express variants). M.2 has expanded rapidly through the laptop SSD world, and today, the SSDs that still use the 2.5-inch form factor are primarily meant for upgrading desktop PCs and older laptops.
Best M.2 SSDs We've Tested...
The Advantages and Disadvantages of SSDs and HDDs
Both solid-state and hard drives do the same job: They boot your system and store your applications and personal files. However, each type of storage has its own unique traits. How do they differ, and why would you want to get one over the other?
How Expensive Is an SSD vs. HDD?
Although they were once prohibitively costly, SSDs are no longer much more expensive than hard drives in terms of dollars per gigabyte. A 2TB internal 3.5-inch hard drive costs around $60, while the cheapest 2.5-inch SSDs of the same capacity start at around $90.
How Roomy Is an SSD vs. an HDD, in Maximum and Common Capacities?
Consumer SSDs come in capacities of up to 8TB, but these are expensive. You're more likely to find 500GB or 1TB units as primary drives in systems. While 500GB is considered a "base" boot drive capacity for mainstream laptops these days, pricing concerns can push that down to 256GB for lower-priced SSD-based systems. Users with extensive media collections or who work in content creation will require even more, with 2TB and 4TB drives available in high-end systems.
Meanwhile, consumer desktop PCs with an SSD as a boot drive and one or more secondary HDDs typically offer HDDs with much greater capacities than the boot SSD. In this scenario, you could buy a desktop with a 1TB SSD boot drive and one or more 8TB or 16TB HDD secondary-storage drives, for example, though you can get individual hard drives in capacities up to 22TB.
How Fast Is SSD vs. HDD?
Speed is where solid-state really shines. An SSD-equipped PC will boot in less than a minute, often in seconds. Hard drives require time to speed up to operating specs, and will continue to be slower than SSDs during everyday use, which is another reason why they are no longer used as boot drives. A PC or Mac with an SSD boots faster, launches and runs apps faster, and transfers files faster.
A secondary issue to this is fragmentation. Because of their rotary recording surfaces, hard drives work best with larger files laid down in contiguous blocks. That way, the drive head can start and end its read in one continuous motion. When hard drives start to fill up, bits of large files end up scattered around the disk platter, causing the drive to suffer from what's called "fragmentation." While read/write algorithms have improved to the point that the effect is minimized, hard drives can still become fragmented to the point of affecting performance. SSDs can't, however, because the lack of a physical read head means data can be stored anywhere without penalty. This contributes to SSDs' inherently faster nature.
Best External HDDs We've Tested...
To put it in number terms? A typical platter-based single hard drive won't exceed read and write speeds of 250MB per second, whereas a basic SATA-bus SSD will routinely peak at transfer rates twice that. Newer-generation PCI Express-based SSDs, depending on the generation and number of PCI Express lanes supported, can be far faster. They can easily top four times the speed of the quickest platter drives, and the fast-becoming-mainstream PCI Express Gen 4 drives can top out at 3,500MB or even 7,000MB per second, depending on the model. (Cutting-edge PCI Express 5.0 drives can exceed five-figure transfer rates.) They won't read and write at those peak speeds all the time, but as you can see, the comparison between the technologies is not even close.
How Durable Is an SSD vs. an HDD?
An SSD has no moving parts, so it is more likely to keep your data safe if you drop your laptop bag or your system gets shaken while it's operating. Most hard drives park their read/write heads when the system is off, but when they are working, the heads fly over the drive platter at a distance of a few nanometers. Besides, even parking brakes have limits. If you're rough on your equipment, a solid-state drive is recommended.
What Are the SSD and HDD Form Factors?
Because HDDs rely on spinning platters, there is a limit to how small they can be manufactured. Years back, there was an initiative to make tiny 1.8-inch spinning hard drives, but that stalled at about 320GB, and they're gone. The standard sizes today are 2.5-inch and 3.5-inch. The 2.5-inch size is meant for older laptops that have a 2.5-inch drive bay; they can also be mounted in desktop PC cases, as modern cases provide mounting points for both drive sizes. The 3.5-inch form factor is designed strictly for installation in desktop PCs. Their platters have a bigger radius, and thus can hold more data at any given density versus 2.5-inch drives.
SSDs can be inherently smaller since there is no platter, spindle, or motor to factor in, so they can continue to shrink as time goes on. As discussed above, SSDs are available in 2.5-inch laptop-drive sizes, but that's only for convenience in fitting within established drive bays. Most modern laptops and desktops come with a gum-stick-sized M.2 SSD attached directly to the motherboard, and many modern desktop boards have multiple M.2 slots. Note, though: In the case of some (usually ultrathin) laptops, the SSD NAND chips are mounted directly on the motherboard and are not upgradable or removable.
Best External SSDs We've Tested...
SSD vs. HDD Noise, Power, and Lifespan
Even the quietest hard drive will emit a bit of noise when it is in use. (The drive platters spin, and the read arm ticks back and forth.) Faster hard drives will tend to make more noise than slower ones. SSDs make no noise at all; they're non-mechanical.
Plus, an SSD doesn't have to expend electricity spinning up a platter from a standstill. Consequently, none of the energy consumed by the SSD is wasted as friction, rendering them more efficient. On a desktop or in a server, that will lead to a lower energy bill. On a laptop, you can eke out more minutes (or hours) of battery life.
Then there's the issue of longevity. Yes, SSDs indeed wear out over time. (Each cell in a flash-memory bank can be written to and erased a limited number of times, measured by SSD makers across the whole drive as a "terabytes written" or TBW rating.) But thanks to TRIM command technology that dynamically optimizes these read/write cycles, you're more likely to discard the system for obsolescence before you start running into read/write errors with an SSD or the PC starts decommissioning overused cells. If you're worried, several tools can tell you if you're approaching the drive's rated end of life.
Eventually, hard drives will wear out from constant use, as well, since they use physical recording methods and motors. Longevity is a wash when it's separated from travel and ruggedness concerns.
SSD vs. HDD Storage: Breaking It Out by Use Case
The overall takeaway? SSDs work best for most consumer use cases. If it weren't for the relative price per gigabyte and the significant differences in peak capacity, SSDs would be the hands-down winners. That said, there are still a few situations where an HDD may be a better choice.
What Are HDDs Best For?
Desktop PCs with multiple storage drives. Enthusiast multimedia users, heavy downloaders, and gamers need lots of storage space, and you can easily get to 8TB or much more space cheaply with one or two hard drives paired with a smaller SSD boot drive.
External drives. Backing up a big data store doesn't require SSD speeds, but it does require a lot of space. HDDs meet both of these requirements.
What Are SSDs Best For?
Pretty much every other computing scenario: for frequently carried-around drives (SSDs are more rugged), in laptops (which tend to get jostled), and in cases where speed matters most (it's no contest).
SSDs: The Best Drives for Most People
Once luxury components or elite parts reserved for enterprises, SSDs are now the go-to storage technology for local data storage. They're no longer prohibitively expensive, though they're still a bit too costly to fully replace dozens of terabytes of data that some users have in their PCs and Macs (in cases where they have mass storage that doesn't need to be fast but just simply there). But as boot drives serving in systems that require only a moderate amount of local storage, SSDs rule.
Hard drives' big draw remains big capacity on the cheap. An HDD may still be a better option for external drives that will sit on a desk or in desktop PCs with massive storage and backup needs across multiple drives.
Looking for some extra storage? Check out our guide to the best external hard drives. Or if you want to protect or store your files online, check out our roundups of the best cloud storage and file-syncing services and the best online backup services.
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