A hard disk drive, also known as a hard drive or HDD, is used to store data long term. Data can include the computer's operating system and applications as well as personal files including photographs, documents and music. When a computer is booted up, or turned on, the computer access the hard drive to load the operating system, such as Windows, OSX or Linux.
The major characteristics of hard drives are the amount of data it can store, called capacity, and performance. The capacity is measured in gigabytes or terabytes. For example, 1 GB is equal to 1,024 megabytes and 1 TB is equal to 1,024 GB. When it comes to selling hard drives, however, the manufacturer usually uses multiples of 1,000 rather than 1,024. This can make some hard drives show less capacity in the operating system than they do on the box.
Hard drive performance can refer to read speeds, write speeds, latency and the seek time. The seek time and latency are measured in the time it takes for the magnetic heads in a magnetic hard drive to access the data desired. The read and write speeds, or data transfer rate, are measured by how much data can be written to the drive or read from the drive within a specified period.