In 1983, International Business Machines (IBM) announces the PC XT computers in New York City. The US$4,995 system features the first built-in 10 megabyes (MB) hard disk in a desktop personal computer running on Microsoft DOS version 2 Operating System.

Since then, for the past two decades, in terms of basic design, today's hard disks still look strikingly similar to their ancestors. However, in terms of storage capacity, reliability, and other characteristics, the hard disks have undergone more change than anyone can imagined.

In fact, modern disks now packs as much as 20GB of data onto a single platter. Not only are the disks getter smaller, the magnetic storaeg systems store much more, cost much less, are much faster and more reliable, but used much less power.

Capacity

When purchasing a hard disk, the first thing to find out is the sizes that are available and which represents the best value, especially for your budget.

Generally, operating systems and software applications hog disk space, the files generated usually do not. Word processing documents, Excel spreadsheets, Powerpoint slides and such are usually relatively small in size.

On the other hand, media files take up much more disk space. An average photo, taken with a 3-5 mega-pixel camera is about 1 MB in size. An average digital music file in MP3 format take up 4MB in size and 4 minutes in length. Video files will take up even more space than music or photo files. A short video clip can easily take up 50-100MB.

Spindle Speed

The move to faster and faster spindle speeds continues. Increases in spindle speed improves random-access and sequential performance. Currently. 5,400 and 7,200 RPM are the standard for mainstream IDE/ATA drives. Higher-end SCSI disks often comes with higher spindle speeds, hence better read/write performance

Form Factor

The trend in form factors is towards smaller and smaller drives. 5.25" drives have now but disappeared from the mainstream PC market, with the 3.5" drives dominating the desktop and server segment. In the mobile world, 2.5" drives are the standard.

Disk Cache

All modern hard disk contains an integrated cache, whose function is to act as a buffer to hold the results of recent reads from the disk. The use of cache improve performance by reducing the number of physical accesses to the disk on repeated reads and allowing data to stream from the disk uninterrupted when the bus is busy.

Interfaces

The interface is the way the hard disk is connected to the PC via a standard plug and cable system. The two common types of internal hard disk interfaces are parallel advanced technology attachment (PATA, which is also know as IDE) and Serial ATA (SATA), while external hard disks use either USB or FireWire. The speed of an interface is only the theoretical transmission rate of the interface itself.

Performance

Hard drive performance is correlated with its mechanical latencies, measured in milliseconds. The "access time" is the interval between the time a request for data is made by the system and the time the data is available from the drive. Access time include the seek time, the rotational latency and command processing overhead time. The "seek time" is defined as the amount of time it takes a hard disk's read/write head to find the physical location of a piece of data on a disk. The "latency" is the average time for the sector being accessed to rotate into position under a head, after a completed seek.