SSD vs. HDD: Know the Difference

Hard disk drives consist of one or more magnetically sensitive platters, an actuator arm with a read/write head on it for each platter, and a motor to spin the platters and move the arms. There is also an I/O controller and firmware that tells the hardware what to do and communicates with the rest of the system.

Each platter is organized into concentric circles called tracks. Tracks are divided into logical units called sectors. Each track and sector number results in a unique address that can be used to organize and locate data. Data is written to the nearest available area. There is an algorithm that processes the data before it’s written, allowing the firmware to detect and correct errors.

The platters spin at pre-set speeds (4200 rpm to 7200 rpm for consumer computers). Those speeds correlate to read/write rates. The higher the pre-set speed, the faster a hard drive will be able to read and write data.

Each time you ask your computer to retrieve or update data, the I/O controller tells the actuator arm where that data is located, and the read/write head gathers the data by reading the presence or absence of a charge in each address. If the request was to update the data, the read/write head changes the charge on the affected track and sector.

The time it takes for the platter to spin and the actuator arm to find the correct track and sector is known as latency.

The drawbacks to HDDs are a result of the mechanical parts used to read and write data, as physically finding and retrieving data takes more time than electronically finding and retrieving data. The mechanical parts can skip or even fail if they are handled roughly or dropped. This is a concern in laptops, but not as much in desktops. HDDs are also heavier and use more energy than comparable SSDs. 

The benefits of a hard disk drivs are that they are a proven technology, and are frequently less expensive than a solid state drives for the same amount of storage. Currently, HDDs are also available with more storage space than SSDs.

Solid state drives use flash memory to deliver superior performance and durability. Because there are lots of small, moving parts inside your hard drive — magnetic heads, spindles, and spinning platters — it's easy for things to go wrong and you could lose your important data. Without moving parts, SSDs are more durable, run cooler and use less energy. 

SSDs can be thought of as large USB drives; they use the same base technology. NAND, the technology in solid state drives, is a type of flash memory. At the lowest level, floating gate transistors record a charge (or lack of a charge) to store data. The gates are organized in a grid pattern, which is further organized into a block. Block size can vary, but each row that makes up the grid is called a page.

An SSD controller performs several functions, including keeping track of where data is located.

Updating data is more complex for SSDs. All the data in a block must be refreshed when any portion of it is updated. The data on the old block is copied to a different block, the block is erased, and the data is rewritten with the changes to a new block.

Each time you ask your computer to retrieve or update data, the SSD controller looks at the address of the data requested and reads the charge status.

When the drive is idle, a process called garbage collection goes through and makes sure the information in the old block is erased and that the block is free to be written to again.

There is another process called TRIM that informs the SSD that it can skip rewriting certain data when it erases blocks. Because there are a finite number of times any block can be rewritten, this is an important process that prevents premature wear on the storage drive.

To further prevent wear on the drive, there is an algorithm to make sure that each block in the drive gets an equal amount of read/write processes. This process is called wear leveling and happens automatically as the drive is working.

Because the read/write process requires data movement, SSDs are usually overprovisioned with storage; there is always a certain amount of the drive that is not reported to the operating system, and not accessible to the user. This allows room for the drive to move and delete items without affecting the overall storage capacity.

SSDs are newer technology, and as such, are more expensive than HDDs. Although they are catching up, it can be harder to find large-capacity solid state drives. HDDs can be as much as 2.5 times larger.

Why choose a solid state drive? SSDs deliver faster load times for games, applications, and movies. Because of the technology they use, SSDs are lighter and better able to withstand movement and droppage. In addition, solid state drives use less energy, allowing computers to run cooler.

One of the biggest benefits of an SSD is how much faster they are than HDDs. For example, the Crucial P5 is our fastest NVMe SSD, delivering impressive read/write speeds up to 3400/3000MB/s. Even portable SSDs are faster than HDDs. With read speeds up to 1050MB/s¹ and capacities up to 2TB, the X8 is up to 100x faster than USB flash drives² and up to 7.5x faster than traditional hard drives².

Compare Crucial solid state drives.

Put simply, an SSD isn’t the same as a hard drive. Hard drives use magnetically sensitive platters which are moved by a motor, whereas an SSD uses flash memory without any moving parts, meaning they are faster.

1. Sequential read speed of 1050MB/s specific to Crucial’s portable X8 drive.
2. MB/s speed measured as maximum sequential performance of device as measured by Crucial on a high-performance desktop computer with Crystal Disk Mark (version 6.0.2 for x64). Your performance may vary. Comparative speed claims measured as maximum sequential performance of similarly situated portable SSDs, mainstream portable HDDs and mainstream USB flash drives from vertically-integrated manufacturers selling under their own brands as of June 2019.