A hard disk drive (HDD), often just called a hard drive, stores data as magnetic patterns on spinning disks. It is the cheapest way to store large amounts of data. Most of the world's data in cloud data centres is still kept on hard drives, even though phones and most laptops now use flash memory instead.
Inside a hard drive are several thin, round disks, called platters, coated with a magnetic layer. They spin at 7,200 revolutions per minute. A head on a moving arm floats a few nanometres above each surface. A nanometre is a millionth of a millimetre. To write, the head magnetises tiny areas of the surface in one direction or the other, which stand for 1s and 0s. To read, it senses the direction of each area. A large data centre drive has about ten platters in a sealed case filled with helium, a light gas that lets the platters spin with less drag. One such drive holds 24 to 30 TB.
The trade-off is speed. To reach a new piece of data, the arm must move to the right track and the platter must turn to the right place. That takes several milliseconds each time. A flash SSD finds data in about a tenth of a millisecond, because it has no moving parts. So hard drives suit data that is written once and then read in long runs, such as video, backups, photos and logs. Data that is read often and in small pieces goes on flash. The oldest data, which is almost never read, often goes to tape, which is cheaper still.
Take, for example, the CCTV control room of a municipal corporation in Pune with 2,000 cameras. Each camera records at about 2 megabits per second. So each camera produces about 21 GB of video a day, and all the cameras together about 43 TB a day. The city keeps 30 days of footage, which is about 1.3 petabytes. A petabyte is 1,000 TB. On 24 TB drives, that needs 54 drives, plus about 15 more so that no footage is lost when a drive fails. At roughly ₹50,000 per drive, the 69 drives cost about ₹35 lakh. The same space on flash would cost several times as much. Each new day's video simply overwrites the oldest day's.
The drives are rarely used one by one. They are fitted into enterprise storage systems, or joined into one pool by software-defined storage. Either way, the software writes every piece of data to more than one drive, so that a failed drive loses nothing. This is called redundancy.
Only three companies make hard drives, and one of them has engineers in Bengaluru. Prices rise and fall with demand, much as they do for memory chips, which is explained on memory chip cycle.