Until recently, PC buyers had very little choice about what kind of storage to get in a laptop or desktop. If you bought an ultraportable, you likely had a solid-state drive (SSD) as the primary drive (C: on Windows, Macintosh HD on a Mac). Every other desktop or laptop form factor had a hard disk drive (HDD). Now, you can configure your system with either an HDD or an SSD, or in some cases both. But how do you choose? We explain the differences between SSDs and HDDs (or hard drives), and walk you through the advantages and disadvantage of both to help you decide.

HDD and SSD Explained

The traditional spinning hard drive is the basic nonvolatile storage on a computer. That is, information on it doesn't "go away" when you turn off the system, as is the case with data stored in RAM. A hard drive is essentially a metal platter with a magnetic coating that stores your data, whether weather reports from the last century, a high-definition copy of the Star Wars trilogy, or your digital music collection. A read/write head on an arm accesses the data while the platters are spinning.

An SSD does functionally everything a hard drive does, but data is instead stored on interconnected flash memory chips that retain the data even when there's no power present. The chips can either be permanently installed on the system's motherboard (as on some small laptops and ultraportables), on a PCI Express (PCIe) card (in some high-end workstations and an increasing number of bleeding-edge consumer systems), or in a box that's sized, shaped, and wired to slot in for a laptop or desktop's hard drive (common on everything else). These flash memory chips are of a different type than is 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.

Note: We'll be talking primarily about internal drives in this story, but almost everything applies to external hard drives as well. External drives come in both large desktop and compact portable form factors, and SSDs are gradually becoming a larger part of the external market.

A History of HDDs and SSDs

Hard drive technology is relatively ancient (in terms of computer history, anyway). There are well-known pictures of the infamous IBM 350 RAMAC hard drive from 1956 that used 50 24-inch-wide platters to hold a whopping 3.75MB of storage space. This, of course, is the size of an average 128Kbps MP3 file today, in the physical space that could hold two commercial refrigerators. The RAMAC 350 was only limited to government and industrial uses, and was obsolete by 1969. Ain't progress wonderful? The PC hard drive form factor standardized at 5.25 inches in the early 1980s, with the 3.5-inch desktop-class and 2.5-inch notebook-class drives coming soon thereafter. The internal cable interface has changed from serial to IDE (now frequently called parallel ATA, or PATA) to SCSI to serial ATA (SATA) over the years, but each essentially does the same thing: connect the hard drive to the PC's motherboard so your data can be processed. Today's 2.5- and 3.5-inch drives mainly use SATA interfaces (at least on most PCs and Macs), though some high-speed SSDs use the faster PCIe interface instead. Capacities have grown from multiple megabytes to multiple terabytes, more than a million-fold increase. Current 3.5-inch hard drives have capacities as high as 10TB, with 2.5-inch drives maxing out at 5TB.

The SSD has a much shorter history. There was always an infatuation with nonmoving storage from the beginning of personal computing, with technologies like bubble memory flashing (pun intended) and dying in the 1970s and 1980s. 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 that we know as SSDs started during the rise of netbooks in the late 2000s. In 2007, the OLPC XO-1 used a 1GB SSD, and the Asus Eee PC 700 series used a 2GB SSD as primary storage. The SSD chips on low-end Eee PC units and the XO-1 were permanently soldered to the motherboard. As netbooks and other ultraportable laptop PCs 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. Other form factors emerged, like the mSATA Mini PCIe SSD card, M.2 SSD in SATA and PCIe variants, and the DIMM-like solid-state Flash Storage in the Apple MacBook Air and MacBook Pro, but today many SSDs still use the 2.5-inch form factor. The 2.5-inch SSD capacity currently tops out at 4TB, but a 16TB version for enterprise devices like servers was released by Samsung in early 2016.

Advantages and Disadvantages

Both SSDs and hard drives do the same job: They boot your system, and store your applications and personal files. But each type of storage has its own unique feature set. How do they differ, and why would you want to get one over the other?

Price: SSDs are more expensive than hard drives in terms of dollar per gigabyte. A 1TB internal 2.5-inch hard drive costs between $40 and $50, but as of this writing, an SSD of the same capacity and form factor starts at $230. That translates into 4 to 5 cents per gigabyte for the hard drive and 23 cents per gigabyte for the SSD. Since hard drives use older, more established technology, they will remain less expensive for the near future. Those extra hundreds for the SSD may push your system price over budget.

Maximum and Common Capacity: Although SSD units top out at 4TB, those are still rare and expensive. You're more likely to find 500GB to 1TB units as primary drives in systems. While 500GB is considered a "base" hard drive in 2016, pricing concerns can push that down to 128GB for lower-priced SSD-based systems. Multimedia users will require even more, with 1TB to 4TB drives common in high-end systems. Basically, the more storage capacity, the more stuff you can keep on your PC. Cloud-based (Internet) storage may be good for housing files you plan to share among your phone, tablet, and PC, but local storage is less expensive, and you only have to buy it once.

Speed: This is where SSDs shine. An SSD-equipped PC will boot in less than a minute, and often in just seconds. A hard drive requires time to speed up to operating specs, and will continue to be slower than an SSD during normal use. A PC or Mac with an SSD boots faster, launches and runs apps faster, and transfers files faster. Whether you're using your computer for fun, school, or business, the extra speed may be the difference between finishing on time and failing.

Fragmentation: Because of their rotary recording surfaces, hard drives work best with larger files that are 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, large files can become 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. SSDs can't, however, because the lack of a physical read head means data can be stored anywhere. Thus, SSDs are inherently faster.

Durability: An SSD has no moving parts, so it is more likely to keep your data safe in the event you drop your laptop bag or your system is shaken about by an earthquake while it's operating. Most hard drives park their read/write heads when the system is off, but they are flying over the drive platter at a distance of a few nanometers when they are in operation. Besides, even parking brakes have limits. If you're rough on your equipment, an SSD is recommended.

Availability: Hard drives are more plentiful in budget and older systems, but SSDs are becoming more prevalent in recently released laptops. That said, the product lists from Western Digital, Toshiba, Seagate, Samsung, and Hitachi are still skewed in favor of hard drive models over SSDs. For PCs and Mac desktops, internal hard drives won't be going away completely, at least for the next couple of years. SSD model lines are growing in number: Witness the number of thin laptops with 256 to 512GB SSDs installed in place of hard drives.

Form Factors: Because hard drives rely on spinning platters, there is a limit to how small they can be manufactured. There was an initiative to make smaller 1.8-inch spinning hard drives, but that's stalled at about 320GB, since the phablet and smartphone manufacturers have settled on flash memory for their primary storage. SSDs have no such limitation, so they can continue to shrink as time goes on. SSDs are available in 2.5-inch laptop drive-sized boxes, but that's only for convenience. As laptops continue to become slimmer and tablets take over as primary platforms for Web surfing, you'll start to see the adoption of SSDs skyrocket.

Noise: Even the quietest hard drive will emit a bit of noise when it is in use from the drive spinning or the read arm moving back and forth, particularly if it's in a system that's been banged about or if it's been improperly installed in an all-metal system. Faster hard drives will make more noise than those that are slower. SSDs make virtually no noise at all, since they're non-mechanical.

Overall: Hard drives win on price, capacity, and availability. SSDs work best if speed, ruggedness, form factor, noise, or fragmentation (technically part of speed) are important factors to you. If it weren't for the price and capacity issues, SSDs would be the hands-down winner.

As far as longevity, while it is true that SSDs wear out over time (each cell in a flash memory bank can be written to and erased a limited number of times), thanks to TRIM command technology that dynamically optimizes these read/write cycles, you're more likely to discard the system for obsolescence (after six years or so) before you start running into read/write errors with an SSD. If you're really worried, there are several tools that monitor the S.M.A.R.T. status of your hard drive or SSD, and will let you know if you're approaching the drive's rated end of life. The possible exceptions are high-end multimedia users (such as video editors) who read and write data constantly, but they will need the larger capacities of hard drives anyway. Hard drives will eventually wear out from constant use as well, since they use physical recording methods. Longevity is a wash when it's separated from travel and ruggedness concerns.

The Right Storage for You

So, does an SSD or HDD (or a hybrid of the two) fit your needs? Let's break it down:

HDDs
• Enthusiast multimedia users and heavy downloaders: Video collectors need space, and you can only get to 4TB of space cheaply with hard drives.
• Budget buyers: Ditto. Plenty of cheap space. SSDs are too expensive for $500 PC buyers.
• Graphic arts and engineering professionals: Video and photo editors wear out storage by overuse. Replacing a 1TB hard drive will be cheaper than replacing a 500GB SSD.
• General users: General users are a toss-up. Folks who prefer to download their media files locally will still need a hard drive with more capacity. But if you mostly stream your music and videos online, then buying a smaller SSD for the same money will give you a better experience.

SSDs
• Road warriors: People who shove their laptops into their bags indiscriminately will want the extra security of an SSD. That laptop may not be fully asleep when you violently shut it to catch your next flight. This also includes folks who work in the field, like utility workers and university researchers.
• Speed demons: If you need things done now, spend the extra bucks for quick boot-ups and app launches. Supplement with a storage SSD or hard drive if you need extra space (see below).
• Graphic arts and engineering professionals: Yes, I know I said they need hard drives, but the speed of an SSD may make the difference between completing two proposals for your client and completing five. These users are prime candidates for dual-drive systems (more on that below).
• Audio engineers and musicians: If you're recording music, you don't want the scratchy sound from a hard drive intruding. Go for quieter SSDs.

Hybrid Drives and Dual-Drive Systems

Back in the mid 2000s, some hard drive manufacturers, like Samsung and Seagate, theorized that if you add a few gigabytes of flash chips to a spinning hard drive, you'd get a so-called "hybrid" drive combining a hard drive's large storage capacity with the performance of an SSD, at a price only slightly higher than that of a typical hard drive. The flash memory acts as a buffer for frequently used files, so your system has the potential for booting and launching your most important apps faster, even though you can't directly install anything in that space yourself. In practice, hybrid drives like the Seagate Momentus XT work, but they are still more expensive and more complex than regular hard drives. They work best for people like road warriors who need both lots of storage and fast boot times. Since they're an in-between product, hybrid drives don't necessarily replace dedicated hard drives or SSDs.

In a dual-drive system, the system manufacturer will install a small SSD primary drive (C:) for the operating system and apps, and add a larger spinning hard drive (D: or E:) for storing files. This works well in theory; in practice, manufacturers can go too small on the SSD. Windows itself takes up a lot of space on the primary drive, and some apps can't be installed on other drives. Some capacities may also be too small. For example, you can install Windows on a SSD as small as 16GB, but there will be little room for anything else. In our opinion, 120 to 128GB is a practical minimum size for the C: drive, with 256GB or more being even better. Space concerns are the same as with any multiple-drive system: You need physical space inside the PC chassis to hold two (or more) drives.

Last but not least, an SSD and a hard drive can be combined (like Voltron) on systems with technologies like Intel's Smart Response Technology (SRT). SRT uses the SSD invisibly to act as a cache to help the system more speedily boot and launch programs. As on a hybrid drive, the SSD is not directly accessible by the end user. SRT requires true SSDs, like those in 2.5-inch form factors, but those drives can be as small as 8GB to 20GB in capacity and still boost performance; since the operating system isn't being installed to the SSD directly, you avoid the drive space problems of the dual-drive configuration mentioned above. On the other hand, your PC will need space for two drives, a requirement that may exclude some laptops and small-form-factor desktops. You'll also need the SSD and your system's motherboard to support the caching technology for this scenario to work. All in all, however, it's an interesting workaround.

The Storage of Tomorrow

It's unclear whether SSDs will totally replace traditional spinning hard drives, especially with shared cloud storage waiting in the wings. The price of SSDs is coming down, but they're still too expensive to totally replace the terabytes of data that some users have in their PCs and Macs. Cloud storage isn't free, either: You'll continue to pay as long as you want personal storage on the Internet. Local storage won't go away until we have ubiquitous wireless Internet everywhere, including in planes and out in the wilderness. Of course, by that time, there may be something better.

Looking for some extra storage? Check out our list of 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.

• BY JOEL SANTO DOMINGO
• NOVEMBER 9, 2016