Hard disks, formerly called hard disk drives and more recently just hard drives, were invented in the 1950s. They are storage devices in desktop computers, laptops, servers, and mainframes. Hard disks use a magnetic recording surface to record, access, and erase data, in much the same way as magnetic tape records, plays, and erases sound or images. A read/write head, suspended over a spinning disk, is directed by the central processing unit (CPU) to the sector where the requested data is stored, or where the data is to be recorded. A hard disk uses rigid aluminum disks coated with iron oxide to store data. Data are stored in files that are named collections of bytes. The bytes could be anything from the ASCII codes for the characters of a text file to instructions for a software application to the records of a database to the pixel colors for an image. Hard drive size ranges from several hundred gigabytes to more than one terabyte.

A hard disk rotates from 5,400 to 7,200 revolutions per minute (rpm) and is constantly spinning (except in laptops, which conserve battery life by spinning the hard disk only when in use). An ultra-fast hard disk has a separate read/write head over each track on the disk, so that no time is lost in positioning the head over the desired track; accessing the desired sector takes only milliseconds, the time it takes for the disk to spin to the sector.

Hard drive performance is measured by data rate and seek time. Data rate is the number of bytes per second that the hard drive can deliver to the CPU. Seek time is the amount of time that elapses from when the CPU requests a file and when the first byte of the file is delivered to the CPU.