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