hard disk drive

An HDD in a nonremovable form of computer data storage. Measured in bits (and subsequently bytes, kilobytes, etc.), it uses magnetism and the binary system to store most of the things that makes your computer work, and your personal files.

It consists of at least one platter, which is a single disk that is rotated around a spindle and read by a head. The speed that the platter can be accessed with is measured in RPM, though a higher RPM is needed as the capacity of the HDD becomes greater.

After a set period of inactivity, the HDD spins down, which means the platters stop rotating to save energy. The drawback is the neccesity to spin back up when you use the HDD again, which takes a few seconds. When the HDD spins down, the head, which is attached to an arm moves from its reading position from over the platter to its resting position which is not over the platter. This is much safer for your HDD, because when you move your computer, or the HDD itself, the arm will not be shaken and damage part of the platter.

Most of the HDD is stored within an air-tight area. This is to prevent dust and any other small particles from entering the main area and ruining the (very sensitive) platters. If the HDD is cracked open, or forcibly opened, it is pretty much guaranteed that the HDD will not work next time that it is used.

Each HDD has a spot which is used to plug a power cable from the power supply, a plug for the ribbon cable (usually EIDE or SCSI) which connects it to the BIOS, and a series of pins that are jumper settings. You can use the jumper settings to set your HDD to either master or slave. This designates its priority on the daisy chain of ribbon cable. Master will give it priority over slave, and this is the preferred setting, usually leaving a CD-ROM drive to be slave.

When first coming out for in-the-home use, HDDs generally had just a few MBs, but have grown amazingly fast (thanks Moore) to now be 80GBs. Thought to be very large now, 80GBs will most likely be small in just a few years. The average price of the newest HDDs has been around $200 for a while now, and will most likely stay that way (at least until spintronics becomes the norm).

A hard disk drive--often called just a "hard disk" or "hard drive"--is a piece of computer hardware intended for data storage. It is usually the boot disk in a computer because of the speed, size, relative cost, and reliability of hard disks (compared to floppy disks, CDs, or optical disks). Most of today's hard disks are mounted internally in either a 3.5 inch or 5.25 inch bay, but legacy and proprietary standards can still be found. Likewise, as the low end of hard disk technology becomes extremely inexpensive, you will increasingly find hard disks inside equipment that isn't strictly for "home computing": a TiVo, a CD "scratcher" turntable, or an X-Box, for example. Hard disks were not always so omnipresent, however.

In 1956, when "computer" meant ENIAC or one of the 46 UNIVACs that had been built (not 46 models--46 units!), data storage was done with punched cards, or if you were lucky, magnetic tape. That year, a company called International Business Machines, looking for a better way, introduced the 305 RAMAC (Random Access Method of Accounting and Control), which stored data on disks. The IBM 350 Disk File could hold 5 million (7-bit) characters--about 4.4M--on its fifty 24-inch platters; access time was remarkably fast, since you could read any portion of the data you wanted to without having to scroll through all the intervening registers of storage. At $35,000 per year to lease the 350 Disk File, this was a steal for large businesses, but not necessarily a home data storage solution. The 350 RAMAC's storage unit was also very bulky by today's standards--the stack of platters was 24" in diameter and almost 36" tall, and was stored inside a cabinet about the size of a fume hood and laboratory bench. Volume was measurable in cubic meters; storage was in the low megabytes; dollar cost was hundreds of thousands. Gordon Moore would be proud1.

The concept of how a hard disk operates has not changed drastically, although there have been a few improvements in the method. The 350 Disk File had one read/write head, which travelled up and down the stack on a vertical rail; modern hard disks have two read/write heads for each platter--one for each side!

Think of the read/write head as the needle in a phonograph or record player, travelling up and down a single spindle with 50 records. If you knew the height, radius, and azimuth of every song, this machine could eliminate the fast forward and rewind hassle of tapes; hard disks did the same thing for data storage years before the CD (when a human controlled the needle arm, vinyl had this feature first). Instead of using a needle to read the data, however, the read/write head used a magnetic pickup, similar to what you'll find inside a microphone or on an electric guitar. It was so sensitive (and so focused) that it could read a variance in the magnetic field emanating from the platter in a specific location without actually touching the surface of the platter.

Instead of deep analog grooves, the platters stored their data digitally. A platter is made from a glass or aluminum substrate, layered with a magnetic medium, and then polished down to very high precision, so the read/write head can "fly" very close to the surface without crashing into the bumps. This hard platter is where the name "hard disk" comes from--it distinguishes such systems from "floppy disks", where there is no substrate. In a modern disk, the operating system will divide each platter into tracks and sectors. A track is an annular region of fixed width--each track consists of all data at a certain radius from the center. A sector is all of the data on one track that falls between two azimuths. Within a sector, there are hundreds, thousands, or millions of individual regions called bits, each of which can be given a magnetic charge to represent a 1 or a by the read/write head.

The other components of a hard drive are its controller and its motors. The controller is a small computer that turns a standardized set of instructions sent from the main computer's CPU into instructions for all of the motors and the read/write head(s). The spindle motor spins up the platters and keeps track of its rotational position, so that it can precisely duplicate an instruction to spin the disk to a given sector millions of times. A modern spindle motor can sustain 7200rpm, and accelerate to that speed in 750 millionths of a second. The arm motor swings the arm(s) with the read/write head(s) back and forth across the platters, to a known radius, again with extreme precision--modern arm motors can go from the spindle to the edge of the platter and back 50 times a second. In older systems, there was also a motor to control height, but now multiple read/write heads have made that motor obsolete.

As hard disks have become smaller, faster, and higher precision, the need for improved quality parts has increased. Faster motors, more precise read/write heads, better data density on platters, and spindle bearings with increased service lives and quieter operation have brought hard disks down into packages measured in cubic inches, storing data in the billions of bytes, and costing customers on the order of tens or hundreds of dollars. Two quick comparisons to the IBM RAMAC: in 1980, Seagate Technology introduced the first hard disk drive for microcomputers, the ST506. It was a full height (twice as high as most current 5 1/4" drives) 5 1/4" drive, with a stepper motor to move the read/write arm up and down, and held 5 Mb; cost to the customer (including a power supply and enclosure!) was over $1,000. As of this writing, Western Digital, a well-known hard disk manufacturer, had just released a 200GB hard drive with fluid bearings, a 7200rpm spindle motor, and a 100 MB/s data transfer rate for $399.3,4,5


1. Yes, I know Moore's Law doesn't say anything about data storage, just memory on silicon, but hard disk storage volume seems to follow a similar progression, and has tied or beaten Moore's prediction for processor density since the early 1980's.
2. Information from (among others) www.ibm.com, www.westerndigital.com, and several excellent tech journals.
3. At the end of the 2004 calendar year, it was possible to purchase a 400GB drive with a transfer rate of 1.5 GB/s (using the serial ATA interface) and a 7200rpm spindle speed for just over $350.
4. At the end of the 2007 calendar year, the same $350 could purchase a 1TB drive with a transfer rate of 3.0 GB/s. The standard interface had doubled in speed, drives were being bundled with up to 32 MB of volatile on-board RAM, and spindle speeds were as high as 15,000 rpm. A comparable drive to the 2004 example had fallen in price to around $60.
5. reserved for future pricing trends

Since something has last been written here, hard disk capacity has, as expected, increased, although much as with processor speeds, the increase seems to have leveled off a little. What hasn't changed, which is also what wasn't mentioned above, and what I am going to write about, is a few of the basic physical facts about hard disks (or hardrives, as others, including myself, would prefer).

For all the changes in computers in the past 20 years or so, some things have stayed relatively the same. One of these is the physical appearance of the hard disk, with a 250 Megabyte hard disk from 1994 and a 250 Gigabyte hard disk from 2006 being virtually identical, if the labeling were to be removed. Almost all IDE hard disks are 3.5 inches wide, by 5 inches long, and weigh about one pound. They are connected to the motherboard of the computer by a cable to carry information, and are connected to the computer's power supply by cables to carry electricity (as opposed to components that use a single connection for both power and data). They also have some pins for jumpers so that the identity of the hard disk as master or slave can be set. (That particular question would better fit under IDE) The hard disk will also have a label, identifying its manufacturer (there are only a half-dozen companies that make hard disks), and also, if you can decipher it, the number of platters, the date of manufacture, and the capacity (which is measured in "decimal gigabytes", not binary gigabytes-another topic best discussed elsewhere). The hard disk will usually have a small controller board attached to its underside, which does all the thinking for the hard disk. The hard disk will also have a number of holes for screws, where the disk is usually connected to the rails or cages of the computer. As noted, except for the date of manufacture and the labeling of capacity, there is no way to know what the hard disk can do from the outside.

There are of course, many exceptions to the general pictures of hard disks. First off, there are early IDE drives that have a much larger form factor. These are museum pieces, however, and shouldn't be worried about. There was also, well into the 1990s, a 5.25 inch wide form factor hard disk, included in many Compaq computers. These were not popular, as they were hard to interchange into other computers, but they are out there. There is also various sizes of SCSI disk, which will often have a different size, and a few different features as far as data connection goes. There are laptop hard disks, which mostly use a 2.5 inch wide form factor, and are of course much smaller and lighter. There is also the new SATA drives, which while roughly the same size as a common IDE disk, have different connectors. And finally, there are always things that are hard to categorize: there are many exotic architectures in computing, and somewhere someone has a hard disk that consists of a bunch of DNA in a tank of nutrient fluid. However, on the whole, hard disks come in a limited amount of shapes and sizes, and change those shapes and sizes slowly over time.

The reason I went to some length to describe this is it often surprises me how little people, even computer people, know about the inside of their computers. Hard disks, like the rest of the inside of the computer, are not just amorphous blobs, but are actual physical objects. A little familiarization with them helps people demystify the inner workings of computers. Also, armed with this knowledge, you can answer the annoying question "How big is your hard drive" with the correct answer: "about 16 ounces".

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