2.2.2 Hard disk drives (HDD)
Hard disks (today generally called hard disk drives or HDDs) have been developed as computer storage devices since the mid-1950s. The prototype of today’s hard disks was the “Winchester” drive developed in 1973. Since the mid- 1980s, technical development has led to ever increasing capacities at continuously declining prices, which has made HDDs the main storage media, both for personal computers and, in particular, for mass storage systems. This development has also led to the decline in the use of recordable optical disks as storage devices after having been popular for audiovisual contents storage in the later 1990s and the early 2000s.
2.2.2.1 Recording principle and components. Hard disk drives consist of one or more rotating disk platters, normally made from aluminium, glass, or ceramics, covered with a thin (10-20 nm) ferromagnetic layer and mounted on a spindle. When in use, present day disks rotate at speeds between 4,200 and 15,000 rpm. A magnetic head, usually one, sometimes several, for each platter, writes and reads the information to and from the magnetic layer.
The head is mounted on a motor driven actuator arm, which permits the fast access of any part of the disk. In order to permit the closest proximity to the disk, yet avoid damage of the magnetic layer, the magnetic head is engineered with an aerofoil shape that allows it to ride on an air (or other gas) cushion that keeps it away from the surface. The head “flies” at a distance of fractions of a nanometre (just a few gas molecules in depth) from the disk surface in order to read short wave length signals. This spacing distance is important to avoid fatal head crashes caused by the head making contact with the surface of the disk. HDDs must not, therefore, be exposed to mechanical shocks during operation.
Until recently, longitudinal recording, comparable to recording on conventional magnetic tape, was standard. Since 2005, perpendicular recording has been introduced which permits higher data densities of three times or more than conventional longitudinal recording.
Originally rotating in air, platters in modern disks are also embedded in a helium atmosphere. The disk is sealed, which protects it to some extent from any intrusion of dust particles. Temperature is also a critical factor: manufacturers quote 40-55°C as maximum temperatures for safe disk operation.
2.2.2.2 Disk sizes. Today, the main sizes of hard disk drives are 3.5 and 2.5 inch. Smaller disks, developed for sub-note books are fading away, if not already obsolete, and are being replaced by solid-state memories.
2.2.2.3 Life expectancy. Life expectancy (LE) of hard disk drives is often referred to by “Mean Time Between (To) Failure” (MTBF or MTTF) which for current products is quoted to be between 1 and 1.5 million hours. However, these figures are extrapolated from laboratory tests and say nothing about the LE of a given medium. More realistic metrics of failure are AFRs, annualized failure rates, which quote the probability of a disk failure, expressed as the percentage of failing disks amongst a large population in relation to their age. Typical values quoted are below 10% for the first five years. Still, neither of these values allows a meaningful prediction of the actual life expectancy of a given individual disk. SMART pre-failure alerts are, however, highly correlating indicators of possible later failures if adequately monitored.
The practical commercial lifetime of a hard disk in a server environment, specifically its economic serviceability, is a key factor. Typically, hard disks are kept in operation between three and seven years.
Debates on HDD shelf storage over longer periods (several decades) have not led to any conclusive results.
To summarize, an individual hard drive is an inherently unreliable data carrier. However, storing multiple copies of each file in a well-managed mass storage system consisting of multiple hard disk drives with self-checking and self-healing protocols is an efficient and safe method of long-term storage (IASA-TC 04, 6.3.14-21).
