Computer Hardware

What is the difference between a hard drive and a solid-state drive?



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Kevin Stringer

Both devices provide non-volatile storage, which means they’re capable of storing data even when a computer loses power (as opposed to something like random access memory, which requires a constant source of electricity).

Hard disk drives (HDDs) use mechanical components to store data using magnets, while solid-state drives (SSDs) store data electronically. SSDs do not contain moving parts, which is advantageous for several reasons.

To understand the benefits of each technology, we need to understand how they function. It’s fascinating stuff, but skip over this next section if you simply want to review the advantages and disadvantages of each.

How Hard Drives Work

Hard drives contain one or more platters, which are discs coated with a thin layer of magnetic material. As the platters spin, the head stack assembly—similar in appearance to a turntable stylus—reads and writes data as north or south magnetic charges. These charges appear to the computer as 0’s and 1’s (a system known as binary data). Each binary digit is called a bit.

Modern hard drives can store an incredible amount of data. A 1-terabyte hard drive contains 8 trillion bits. That’s a lot of zeros, and it means the surface of a hard drive’s platters are densely packed with information.

The hard drive’s read/write heads never actually touch the magnetic material; they float on a cushion of air generated by the spin of the platters. That cushion of air is incredibly small, about 5 to 10 nanometers. To put that in perspective, a human hair is about 90,000 nanometers in diameter.

If something goes wrong, the heads could come into contact with the platters, scraping off the magnetic material and physically removing data from the hard drive. That’s called a head crash, and it’s one of the most common sources of data loss. That’s where phrases like “my computer crashed" came from.

Modern hard drives have plenty of safeguards to prevent this type of catastrophic failure. Still, because they use moving parts, every hard drive eventually fails—the spindle that moves the platters could seize, the heads could become misaligned, or the circuit board that relays data to the rest of the computer could stop working.

HDD read/write speeds are also limited by the speed of their spindles. Typical computer hard drives spin at 7,200 rotations per minute or faster, but even so, to read a certain bit, the heads must physically move over that space on the platter.

How Solid-State Drives Work

As their name implies, solid-state drives don’t rely on moving components to store data. They consist of NAND flash memory, which uses floating gate transistors to store data with electricity.

If that sounds complicated...well, it is, but the basic function is fairly simple. Basically, the floating gates register as a “0" when they hold an electric charge and as a “1" when they don’t hold a charge. The gates are organized into smaller pages and larger blocks.

Here’s the main drawback of the technology: SSDs can’t really modify data easily. To change a file, the drive needs to make a copy of an entire block, erase the first block, then rewrite it with the new data. Erasing a block of data requires a relatively large amount of voltage, so overwriting data is a complex process, and floating gates become less reliable as they’re overwritten multiple times.

What does that mean for your data? Eventually, your SSD will become slower, and data corruption will become more common. Modern SSDs have technology to limit this effect, but that’s outside the scope of this answer—the important point to keep in mind is that every type of long-term data storage device will eventually fail, so while solid-state drives are more reliable than traditional hard drives in some respects, they’re not perfect.

Should you use a hard drive or a solid-state drive?

Both storage technologies have benefits. Depending on how you use your computer, you might prefer one type of storage.

Here are some of the advantages of each option:

  • Hard drives are (currently) larger and less expensive. At the time we wrote this, SSDs cost about twice as much as hard drives on a per-gigabyte basis. For the average computer user, HDDs also offer more capacity. If you’re storing a ton of files and you’ve got a limited budget, you’ll probably want a hard drive. That might change over time. The per-gigabyte price of solid-state drives has dropped in recent years and should continue to drop as more consumers adopt the technology.
  • Solid-state drives are less susceptible to physical damage. Because they don’t have moving components, solid-state drives are often a better choice for laptops and external storage. Of course, other computer components are still susceptible to physical damage, so don’t take this as an excuse to throw your devices around.
  • Solid-state drives are much faster. A typical SSD can read data at around 550 megabytes per second (MBps) and write at 520 MBps. A hard drive might achieve speeds of around 125 MBps. If you’re concerned with performance (for instance, if you’re building a gaming PC), a solid-state drive offers huge speed advantages.
  • Solid-state drives generally use less power than hard drives. When spinning up, a hard drive can use as many as 25 watts. While writing data, most SSDs use fewer than 2 watts. That’s important if your device relies on battery power. If you’re buying a drive for a laptop and you’re concerned about battery life, an SSD might offer a significant advantage (but be sure to check the SSD’s wattage consumption before making your purchase).
  • When you delete data from a solid-state drive, it’s probably gone for good. When you delete data on a hard drive, the device doesn’t overwrite your files right away; it simply marks the space occupied by the files as available, then writes over that part of the platter later. Because of how they read and write data, solid-state drives clear out the space almost immediately. In other words, if you delete a file from a hard drive using conventional methods, it will be recoverable for some time. That’s not true of SSDs. This isn’t a major concern for most computer users, but it’s an important point to keep in mind if you regularly work with sensitive data.

Regardless of which storage device you choose, you should make sure you have a decent backup plan in place. Never trust a single device with all of your important data, and try to keep some free space on your HDD or SSD to allow it to function as efficiently as possible. If your computer seems much slower than normal, or if you hear sounds coming from a hard drive, back up important files immediately and replace your storage media.

While neither technology is perfect, they’re both capable of storing incredible amounts of information, and they’re both reliable options when used properly.