Hard Disk Drives are sometimes referred to as hard drives, hard disks, or HDD for short. Since the phrase hard drive (plural, hard drives) is by far the most popular, I will be using it more often to refer to a hard disk drive.

Hard drives are electro-mechanical storage devices used for storing data and program files. Every personal computer (PC) comes with at least one hard drive. This hard drive is responsible for storing the operating system (OS) software (Microsoft Windows, for example). Stored along with the OS are application softwares and user data files such as Word documents, digital pictures, video and music files, and others.

A hard drive is composed of several rigid, non-magnetic platters (disks) coated with magnetic materials that serve as the storage medium. The disks are attached to a spindle that rotates at a very high speed. The rotating disks and their associated mechanism are enclosed in a sealed metal container.

Hard drives can be classified into either an internal drive or an external drive. Internal drives are designed to be installed inside the computer case. External drives stay outside the case like a plug-in accessory.

Internal hard drives are further classified based on there interface, that is, the way they are connected to the computer. A SATA (Serial Advance Technology Attachment) hard drive uses a SATA cable, while a PATA (Parallel Advance Technology Attachment) hard drive uses an entirely different PATA cable. The PATA interface is also known as an IDE (Integrated Drive Electronics) interface.

External hard drives are also of two types. It's either a desktop or portable drive. Portable drives are smaller in size than a desktop drive and they are designed to be hot pluggable, meaning you can plug and unplug them even when the computer is powered. Portables are designed to be carried around and plugged into another computer.

When shopping for a new hard drive, there are several things you would have to consider before actually buying one.

For internal drives, make sure you have an availabe interface slot on the motherboard. Old motherboards will accept only IDE drives. Newer motherboard models usually have several SATA ports in addition to an IDE (PATA) port.

External drives, generally, have no problem with regards to the interface because they use the USB port or a firewire port that is available in many computers.

Hard drives have different disk space capacities which are measured in Gigabytes (GB) and Terabytes (TB). 1000GB is equal to 1TB. The higher this number is, the more program and data files can be stored. If you are curious just how much information a one (1) TB hard drive can hold, one byte is roughly equivalent to one character of a Word document file. It can hold approximately 1,000,000,000,000 characters of your written text.

Hard drives have a spindle speed that can vary from model to model. The higher the speed is, the faster the hard drive can read and write information from its storage medium.

In order to further increase the performance of hard drives, manufacturers have more recently implemented a cache memory on the drives. Frequently used data from the hard drives are stored in the cache memory. When the CPU requests for a data from the hard drive, the hard drive checks the cache memory first for the requested data, and if it finds it there, the data is immediately sent to the CPU without physically reading the information from the storage media. This technique greatly enhances the performance of the drive. The higher the cache memory is, the better.

I have included below examples of hard disk drives from Seagate, one example each for the different types as discussed above. The following information are from Amazon.

SEAGATE BARRACUDA IDE HARD DRIVE

• Interface: ATA
• Cache: 8MB
• Guaranteed Sectors: 781,422,768
• Number of pins on the I/O connector: 40
• Spindle Speed: 7200 rpm

Seagate Barracuda 7200.8 drives provide high-performance, highly-reliable storage for PCs and advanced applications, backed up by an industry-leading five-year warranty. Their best-in-class combination of capacity, performance, and support ideally suit them for such applications as high-performance PCs, media PCs, ATA-based servers and RAIDs, and cost-effective network attached storage (NAS)

SEAGATE BARRACUDA SATA HARD DRIVE

• 5,900 RPM Spin Speed, 32 MB Cache Buffer, SATA 3Gb/s interface, 2 TB Capacity
• Super-low power consumption reduces power costs over the life of the system up to 50% over those of standard desktop drives
• Leading performance in the low power category
• Cool drives provide long-lasting benefits to the PC or external drive, reducing some component and maintenance costs.
• Built to exacting Seagate green standards: Complies with RoHS directive, typically 70 percent or more of the materials used to build the drive can be recycled and designed, built and delivered using best-in-class, environmentally friendly processes.

SEAGATE EXTERNAL DESKTOP HARD DRIVE

With Seagate® External Desktop drives, you can easily and instantly add more storage space to your system. There’s no software to install—they’re automatically recognized by Windows Vista and Windows XP. Simply drag and drop to save files. Fast USB 2.0 performance makes saving and transferring large music and video collections quick and painless. Seagate External Desktop drives make “going green” easy too, with built-in power management that ensures energy-efficient operation. And they’re backed by a 2-year limited warranty for peace of mind.

SEAGATE FREEAGENT GO

• 5,400 RPM Spin Speed, 8 MB Cache Buffer
• USB 2.0: up to 480 Mb/s
• Sync and back up quickly at your desktop without having to search for cables.
• Guard the privacy of important files and folders.
• Get your storage solution up and running quickly.

With its sleek design and huge capacity, the FreeAgent® Go portable storage solution makes it easy to take your photos, music, videos, and documents everywhere. Included, easy-to-use software helps keep your information safe with automatic data backup and encryption of your important files and folders. You can sync content between your work and home computers so you’ll always have your most current versions. And you can take advantage of the world’s first portable drive with a convenient desktop dock accessory. Simply pop your drive into the FreeAgent dock whenever you want to access your files — no more fumbling for cables!

Winner of CNET Editors’ Choice, September 2008 and PC Magazine Editors’ Choice, October 2008.