Hard disk drives

Hard disk drives (HDD) comprise an assembly stacking up to 12 (typically 5 or 7) rigid platters coated with magnetic oxide, each capable of storing data on both sides. Each recording surface has an associated read/write head, and any one may be activated at a given instant. Disk drives give rapid access to vast amounts of data, and are highly reliable as they have only two moving parts – the swinging head assembly and the spinning disk. They can be written and read millions of times. The use of disks to store audio, video and images has changed many aspects of digital production editing and transmission.

For high capacity, disks pack data very tightly indeed. Areal density, the amount of data stored per unit area of the disk surface, is one measure of their technology. Currently available high capacity drives from manufacturer Seagate achieve nearly 1 Tb/square inch in a 6 TB drive. Several new technologies are ready to boost the performance of future drives. These include filling the drive with Helium gas offering lower friction, so allowing more platters to be used in the same standard sized 3.5-inch case. Perpendicular magnetic recording (PMR – laying tracks under one another) is established technology. Upcoming techniques include shingled magnetic recording (SMR) and heat-assisted magnetic recording (HAMR). These create new spaces for yet greater capacities and performance of disk drives.

The rate of increase in capacity seems to have slowed in recent years which may mean that the currently used technology (PMR) is nearing its practical limit. For this performance the heads float only a few molecules off the disk surface, so that even minute imperfections in the surface can cause heating of the head assembly. As a result, high capacity disk drives have to be handled with great care, especially when running. Vibration could easily send heads off-track or crashing into the disk surface – possibly with terminal consequences.

Where will hard disk drives be in 10 years’ time? In 2000, when 50 GB was ‘high capacity’, the Digital Fact Book successfully predicted the arrival of 2TB drives in 2010. Now a new 10-year prediction is made. The long term historic HDD storage development doubles the capacity every two years (increasing 41%/year). In some years it has reached 60%, but the DFB believes progress is getting tougher and expects progress to be nearer 41%/year. Anyway, who wants a 660 TB HDD – perhaps someone working with 8K UHD at 120 f/s?

HDD capacity chart























See also: Areal Density, RAID, SCSI

Website: www.seagate.com