HDD vs. SSD: How They Work, and When To Use Them

HDD vs. SSD: which should you put in your PC? While you might not see any difference when saving and moving files around on computers that use either traditional hard drives or SSDs, the two pieces of hardware couldn’t be more different in how they function. This general guide should inform how they work, and what you should use the drives for. Not all storage solutions are created equal for the same use case.

HDD

                                                                              

This is the inside of your traditional hard drive, and it still uses the same style of electromagnetic tech that makes floppy disks and credit card strips work.  A series of “platters,” or disks coated in a magnetic material, are rotated at high speed, while the read/write arm follows commands to read or write information, providing you with everything from programs to documents. Reading and writing happens by slightly changing the magnetic pattern on the surface of the platter. One magnetic polarity correlates to “1,” and the other to “0,” so the disk can write and read in binary code.

So what does this type of storage look like? Picture the capacity of your hard drive like a spreadsheet, like the picture below:

When you save files, the hard drive magnetically writes the binary code for them into storage cells:

This way, the drive can read the code later and pull the document up again.

The only commands that a traditional hard drive has are “read,” or “write.” Notice that there is no “erase” or “delete” command. So what happens when you “delete” a file?

If you delete a file on your computer, you are telling the drive to treat the space like blank space. It’s the same with reformatting a drive; essentially, you’re just ignoring the file or files until the hard drive fills that space with new information.

Because of this, the information is still there for a while, waiting to be overwritten. This is why files on hard drives are often recoverable even after they’ve been “deleted,” assuming of course that they have not been overwritten already.

Advantages of HDDs

• Reliable over long periods of time (9-11 years if treated well)
• Best choice for archiving information and mass storage, as they resist degradation over time if kept in a dry environment with a low temperature.
• They are generally cheaper than SSDs
• The risk of sudden, catastrophic data loss is typically lower
• Great solution if you’re focused less on performance and more on reliable function.
• Of the two kinds of storage, they are far easier to reliably erase and reuse, since there are standardized methods of doing so (ATA Secure Erase being the most common).

Disadvantages

• HDDs can be subject to mechanical failures due to their many moving parts
• Slower response times
• Sensitive to temperature, and can degrade if left in ambient temperatures over 95° F (35° C). They will need time to acclimate if moved from cold to warm environments, and vice versa.
• Weak to vibration and shock
• Heavier and bulkier

RELATED: How To Buy Storage in 2019

SSD

 

SSDs are fundamentally different than HDDs in nearly every aspect of how they store data, though you might not notice a difference as you save, delete, and move files.

Unlike an HDD drive, most SSDs store and retrieve data via using only electronic circuits, without the use of any mechanical parts. Typically, they use “non-volatile flash memory,” meaning that the device will retain data even if you turn off your computer, but others will have volatile memory similar to RAM, and use batteries to maintain power and retain their data. Modern SSDs commonly use NAND non-volatile flash memory.

So how does Flash memory work? Like its predecessor, the drive has read and write functionality, but it also has the ability to erase data. However, this doesn’t mean that your files are removed when you hit “delete.” Let’s go back to our spreadsheet example:

Rather than creating a magnetic pattern, the data is electronically programmed into the storage cell. These cells are organized into “pages,” and typically 64 pages are grouped into a “block.” Blocks can only be written into once fully, and then they have to be completely erased and re-written.

 

The problem is, each time an electronic command is sent to the SSD, a small amount of physical wear happens to the drive. If you erased and re-wrote data to the same block over and over, the drive would wear out far faster than its true lifespan. To counteract this, “wear leveling” is utilized to write data evenly over the drive. Your data was told to be erased, but rather than write over the old place, new data is written to a new block:

 

This turns traditional data overwriting into a strange game of musical chairs, and it might be a longer period of time before that free space is erased and replaced with new data. Copies of “erased” data may float around in your SSD for some time.

Additionally, SSDs tend to “over-provision” their data, to further extend the life of the device:

This additional space is used in wear leveling, but it is often inaccessible to the consumer or even to data erasure software, due to the freeze locks placed on it. This is something to consider if you plan to wipe and reuse the drive.

Advantages of SSDs:

• SSDs are far faster than HDDs, and even cheap models outperform traditional drives. Windows will boot up faster, programs will load sooner, and files will save more quickly. Cheaper drives tend to have lower write speeds, while more expensive models have more equal read and write speeds.
• Very low risk of failure due to movement, shock, and contamination.
• Will operate in more extreme temperatures, altitudes
• Very high storage capacity

Disadvantages

• Without power, worn SSDs will begin to lose data within one or two years. SSDs are not recommended for archival use.
• More expensive than traditional drives.
• Reliability varies significantly from brand to brand, model to model.
• Sensitive to power outages

RELATED: Our 2019 SSD Buying Guide

HDD vs. SSD: Why not both?

It is entirely possible, and recommended, that you use both types of drives in your computer. Modern motherboards have plenty of options to mount an m.2 SSD and several traditional drives, for example. Hybrid drives also exist, although typically these are best used in laptops.

Where the SSD will provide you with near-instant access to your programs and documents, the HDD will store large amounts of data for longer periods of time. This puts you in the sweet spot of performance and reliability, giving you a machine built to last a long time.