SSD Vs. Hard Drive Speeds

By David Perez

The read/write magnet and disk of a hard drive exposed
i Jupiterimages/ Images

Solid state drives consistently outperform hard disk drives in terms of speed. This is a direct result of their basic differences in their construction. Hard drives contain mechanical components that must physically move in order to locate information. In contrast, solid state drives have no moving parts. Thus they transfer data as quickly as their circuitry can carry it. The real-world speed difference depends on the tasks being performed and is limited by the interface that allows the drive to send information to other computer components.


To understand the difference between HDD and SSD speeds, it's necessary to look beyond the normal specifications given for HDD speeds. Manufacturers commonly report HDD speeds in terms of rotations per minute. This illustrates how fast the drive's disks spin. Since SSDs do not use rotating disks, the best way to compare them is in terms of the amount of data they can find and the amount they can save in a give amount of time. That is, read time and write time. Lower-end HDDs read around 100 megabytes per second and write about 40MB/s. Higher end HDDs read at least 180MB/s and can generally write 60MB/s or more. Compare this to read and write speeds of 200MB/s and100 MB/s, respectively in the low-end SSDs. At the high end, SSDs can read at least 550MB/s and write 500MB/s or more.

Mechanical Differences

HDDs store their data on magnetic disks stacked on a spindle and include an electromagnet on a swivel arm. The drive locates information by rotating the spindle and changing the angle of the arm to align the desired data with the magnet. The magnet then does the reading and writing. Data farther away from the magnet takes longer to access than data closer to it. Solid State drives contain an array of transistors. These are electrical switches that can be either on or off, thus representing either a 1 or a 0, or a bit of data. Since there are no moving parts, SSD drives take the same amount of time to locate any piece of date no matter what its physical location in the memory.

Real-world Performance

SSDs tend to achieve their highest speed advantages over HDDs during the computer's initial startup, when installing new software and when copying files to or from the drive. Hard drives also tend to slow down over time. This is because the data that composes their files gets spread out over the physical space on the disk. This requires the disks to spin to multiple locations to get the data for a single file. This is know as "fragmentation." While it can be solved by running a "defragmentation" program, SSDs do not have this issue at all.


Both SSDs and HDDs require a physical link send and receive information. This link has its own speed restrictions completely separate from those inherent to the drive. Both types of drive use either version two or three of the Serial Advanced Technology Attachment. Version two limits data transfers to 300MB/s while version three limits them to 600MB/s. No matter what the speed of the drive, if it uses SATA II or SATA III, it won't transfer data at any speed beyond these limits.