SEM VI BCM Hard Drive(Part-2)
The platters are the most important parts of a hard drive. As the name suggests, they are disks made from a hard material such as glass, ceramic, or aluminum, which is coated with a thin layer of metal that can be magnetized or demagnetized. A small hard drive typically has only one platter, but each side of it has a magnetic coating. Bigger drives have a series of platters stacked on a central spindle, with a small gap in between them. The platters rotate at up to 10,000 revolutions per minute (rpm) so the read-write heads can access any part of them.
There are two read-write heads for each platter, one to read the top surface and one to read the bottom, so a hard drive that has five platters (say) would need ten separate read-write heads. The read-write heads are mounted on an electrically controlled arm that moves from the center of the drive to the outer edge and back again. To reduce wear and tear, they don't actually touch the platter: there's a layer of fluid or air between the head and the platter surface.
Reading and writing data
The most important thing about memory is not being able to store information but being able to find it later. Imagine storing a magnetized iron nail in a pile of 1.6 million million identical nails and you'll have some idea how much trouble your computer would get into if it didn't use a very methodical way of filing its information.
When your computer stores data on its hard drive, it doesn't just throw magnetized nails into a box, all jumbled up together. The data is stored in a very orderly pattern on each platter. Bits of data are arranged in concentric, circular paths called tracks. Each track is broken up into smaller areas called sectors. Part of the hard drive stores a map of sectors that have already been used up and others that are still free. (In Windows, this map is called the File Allocation Table or FAT.) When the computer wants to store new information, it takes a look at the map to find some free sectors. Then it instructs the read-write head to move across the platter to exactly the right location and store the data there. To read information, the same process runs in reverse.
How does an electronic computer manipulate all the mechanical nitty gritty in a hard drive? There is an interface (a connecting piece of equipment) between them called a controller. This is a small circuit that operates the actuators, selects specific tracks for reading and writing, and converts parallel streams of data going from the computer into serial streams of data being written to the disk (and vice versa). Controllers are either built into the disk drive's own circuit board or part of the computer's main board (motherboard).
With so much information stored in such a tiny amount of space, a hard drive is a remarkable piece of engineering. That brings benefits (such as being able to store 500 CDs on your iPod)—but drawbacks too. One of them is that hard drives can go wrong if they get dirt or dust inside them. A tiny piece of dust can make the read-write head bounce up and down, crashing into the platter and damaging its magnetic material. This is known as a disk crash (or head crash) and it can (though it doesn't always) cause the loss of all the information on a hard drive. A disk crash usually occurs out of the blue, without any warning. That's why you should always keep backup copies of your important documents and files, either on another hard drive, on a compact disc (CD) or DVD, or on a flash memory stick.
Photo: The read-write head on a hard-drive. 1) The actuator arm swings the head back and forth so it's in the right position on the drive. 2) Only the tiny extreme end part of the hard drive actually reads from and writes to the platter. Bear in mind that half of what you're seeing in the second photo is a reflection in the shiny hard drive surface!
Who invented the hard drive?
Like many innovations in 20th-century computing, hard drives were invented at IBM as a way to give computers a rapidly accessible "random-access" memory. The trouble with other computer memory devices, like punched cards and reels of magnetic tape, is that they can only be accessed serially (in order, from beginning to end), so if the bit of data you want to retrieve is somewhere in the middle of your tape, you have to read or scan through the entire thing, fairly slowly, to find the thing you want. Everything is much faster with a hard drive, which can move its read-write head very quickly from one part of the disk to another; any part of the disk can be accessed as easily as any other part. The first hard drive was developed by IBM's Reynold B. Johnson and announced on September 4, 1956 as the IBM 350 Disk Storage Unit.
IBM engineers also pioneered floppy disks, which were removable magnetic disks packed in robust plastic cases (originally 20cm or 8in in diameter and wrapped in flexible plastic sleeves; later 133mm or 5.25in in diameter and packed in tough plastic cases). Developed by IBM's Warren Dalziel in 1967 and first sold in 1971, they became hugely popular in microcomputers (the forerunners of PCs) in the late 1970s and early 1980s, but are now obsolete. With a storage capacity of only 1.44MB, they've been completely superseded by USB flash "drives" that offer hundreds or thousands of times more memory in a tiny plastic stick a fraction the size.
Artwork: The original hard drive. IBM engineers developed this groundbreaking magnetic memory (which, in IBM-speak, was called the DASD, pronounced "das-dee"), through a process of continuous improvement from the early 1950s onward and were awarded their final patent on the design in 1970. You can see that the basic read-write mechanism is exactly the same as in today's drives: there are multiple platters (light blue) made up of individual sectors (dark blue) that can be written to or read from by multiple read-write heads (red) mounted on the ends of sliding actuators (orange). The platters are spun by a pulley and motor (green), while the actuators are driven by gears and a motor (yellow). The main difference between this drive and a modern one is the amazing amount of intricate machinery this one contained (which you can read all about in the original patent). From US Patent 3,503,060: Direct access magnetic disc storage device by William Goddard and John Lynott, IBM Corporation, March 24, 1970, courtesy of US Patent and Trademark Office, with colors added for clarity.
Hard drives and SSDs compared
Hard drives are tried and tested, high-capacity, and cheap, but they have plenty of drawbacks too. One issue is the amount of time it takes for the read-write head to get itself to the right part of the disk to access the information you want. The heft of a hard drive and its relatively heavy power consumption are also problems, especially in mobile devices such as tablets and smartphones. Reliability is another issue. As you'll have gathered from what you've just read, a hard drive is a wonderful bit of precision engineering with plenty of intricate moving parts. It could easily work for 20 years with no problems at all. Then again, if you've ever suffered a hard-drive head crash (a serious mechanical breakdown caused by something like dirt on one of your hard-drive platters or a sudden mechanical shock), and lost everything you've ever stored on your computer, that's no reassurance: you'll know a hard drive will instantly fall out of love with you if you treat it with less care than it deserves.
All these problems—weight, power consumption, access times, and reliability—can be solved by using solid-state drives (SSDs), which typically use flash memory chips instead of spinning magnetic platters. Computer makers have been moving away from hard drives, and toward SSDs, for at least the last decade, largely driven by the trend away from desktop computers and toward mobile devices. Apple iPods are a good example of how times have changed. The original "Classic" iPods, launched in 2001, are little more than hard drives, sound cards, and batteries (you can see what an iPod hard drive looks like in the photos above); the hard drive, in particular, was an obvious excuse for failure if you took them jogging or tossed them around in your bag. With the iPod Touch, which launched in 2007, Apple switched decisively to SSD technology, making music players thinner and lighter in your pocket, less prone to mechanical failure, and giving far better battery life. You're more likely to wear out the buttons or crack the screen on a modern iPod or iPhone than do any damage to the memory chips inside.