Hard drives, once known as ROM or Read Only Memory, have a long and hallowed place in the history of hardware. Although flash memory like the type we use for RAM has always been faster and harder to break, hard drives have doubled in areal density (and thus, the amount of data that can be stored on a single hard drive) every two to four years, making it a hard deal to beat. Only now that storage capacities have grown to a level where many users don’t even need what is available are we seeing flash memory as a main storage media, and even now, it’s mostly the case on small devices like netbooks and cameras. If you have significant secondary storage needs, hard drives are the way to go.
If anything argues against hard drives, it’s their mechanical complexity. A hard drive consists of one or more rotating platters, and an arm with a read/write head that moves to various locations on the disc/s as needed. An important feature of hard drives is that they must remain in their sealed protective enclosure, as the mechanical moving parts are extremely precise and dust would corrupt the disc immediately. The platters themselves are made of a material like glass or aluminum which isn’t magnetic, and then coated with 10 or 20 nanometers of magnetic material and another layer of protective coating. How fast the platters spin is one of the factors in how good a hard drive is, because this impacts the speed of data retrieval. Othe factors in hard drive quality (besides, obviously, data density and size) include how much noise it makes and how shock resistant it is.
The first hard drive was made by IBM in 1956, and could store 3.75 megabytes of information. Now, the largest hard drives are measured in terabytes.