Hard Drives

A hard disk drive is the data file cabinet inside your PC. All the programs on your computer are stored in the hard drive. In addition, whenever you create a document and click "Save," the computer stores that document on your hard drive. The document is written onto hard disks, which look like CDs, and then given an address so that it can be found again.

It's important not to confuse a computer's hard drive and its main memory (RAM). A computer's memory only holds information for the short term. As soon as you turn your computer off, (or the power goes out!) all the information in memory disappears. That's why when you're working it's important to periodically save your data to the hard drive.

The hard drive plays an important role in your system's performance. It is the only integral component that relies on mechanical moving parts to operate. That means it is usually the slowest part of a computer and can hold the whole team back. By upgrading your hard drive you should notice improved speed when booting up your PC, saving and retrieving documents, processing large amounts of data, and multitasking.

The most important aspect of any hard drive is its reliability. But, although reliability is improving all the time, it's not something that can be determined infallibly. So, no matter what drive you purchase, be sure to back up your important data.


Terminology
A hard drive's hard disk(s) are commonly referred to as platter(s). Usually there are between one and four platters. They are stacked on a central spindle, and all move in unison. Each platter is coated with a special magnetic material, and usually both sides are used for storing information.

Electromagnetic read/write heads "write" (record) magnetic patterns onto the platters, or "read" them off of the platters. These patterns can be erased and rewritten at any time. Although only one read/write head can write at a time, they all move in unison.

Each platter is divided into tracks and sectors. The tracks on a platter are like the yearly rings on a tree. A modern hard drive has tens of thousands of tracks on each platter. Each track is divided into smaller parts called sectors, which can hold 512 bytes of information each. Because the outer tracks of the platter are longer they contain more sectors and thus more data. Also, since sectors are so small, they are commonly referred to in groups called clusters .

The more tracks and sectors that are crammed onto a disk, the higher the areal density. The more sectors there are per track, the more information the drive can read in one rotation of the disk. Translation: better performance.

The read/write heads are the most expensive part of a hard drive to manufacturer. When the drive isn't running the read/write heads rest in a special landing zone, which doesn't contain any data and also helps protect the heads during transport. The read/write heads never actually touch the platters. When the platters start spinning, a rush of air lifts them, and they fly over the platters at a height between 5 and 12 millionths of an inch (in modern drives)! That's why some parts of a hard drive have to be manufactured in special clean rooms. Just one spec of dirt landing on a platter could cause a hard drive to crash.

In addition to the platters, a hard drive has one other small, data holding area called the cache buffer. The cache buffer is made of memory. In the case of a "write," data is transferred from the motherboard to the hard drive's cache buffer. From there, the data is transferred to the drive's platters and written onto tracks. In the case of a "read," data is transferred from the platters to the cache buffer, and then transported to the motherboard. In both cases, the cache buffer serves as a pickup and delivery point for data.

In addition, the cache buffer serves one other purpose. Data that has been accessed recently is also kept in the cache buffer temporarily. If that data happens to be requested again soon, the hard drive has instant access to it. This saves time by eliminating the need to retrieve the data from the platters.

Because researchers found only very minimal performance gains between drives with 2MB buffers and 1MB buffers, they concluded that a larger cache buffer didn't really offer any real world performance benefits. However, that idea changed with the advent of Western Digital's Caviar WD1000BB Special Edition hard drive with 8MB buffer. When comparing the performance of Western Digital's WD1000BB with 2MB buffer and Western Digital's WD1000BB Special Edition with 8MB buffer, researchers from StorageReview.com found that the 8MB drive performed much better than the 2MB drive in a number of tests. This led them to draw the following conclusion:

"An increase in buffer size, properly coupled with a well-conceived caching strategy, yields dramatic improvements in performance." -- Eugene Raw, StorageReview.com, "Revisiting the Caviar WD1000BB and the 'Cuda ATA IV," 10/8/01


What Happens When You Click "Save"?
Pretend you are writing a letter in Microsoft Word. Here's a very simplified version of what happens when you click the "Save" command.

Microsoft Word tells the operating system (such as Windows '98) to store your letter in the hard drive.
Your letter, which is currently hanging out in the computer's main memory (located on the motherboard), hops on the hard drive interface, and rides it to the hard drive's cache buffer.
Upon arriving in the cache buffer, your letter is met by the hard disk controller.
The hard disk controller receives a communication from the operating system: "Controller, please store the document at this address: ****."
The read/write heads move into position over the correct track and wait for the correct sector to pass under. As soon as it does, the elected head starts writing the letter onto the elected platter.
But what if the document is really huge and can't fit onto one sector? If the next sectors on the track are empty, the drive will continue writing the document there. If the track fills up and there is still more writing to be done, however, the drive's first choice will be to switch heads. Since the heads all move in unison, all the other heads are positioned over the same track on their respective platter surfaces. It is faster to use an alternate head than to reposition the heads over a new track. A sequential group of tracks is referred to as a cylinder, because they are one on top of another, not side by side.

This brings up another interesting point. After you've owned a hard drive for a while, the availability of long sequential platter space grows scarce. Things start getting messy as you throw files away and add new ones. When this happens your hard drive is said to be fragmented. As a hard drive becomes more and more fragmented it loses efficiency because it is forced to spend more time repositioning its read/write heads. However, you can do what's called defragmenting your hard drive. You just need a defragmenting program, which isn't expensive. The program will reorganize the data on your hard drive for optimum efficiency.


What to Consider
So now that you have a little background on hard drives how do you go about figuring out which one to buy? First of all, it is good to know that different drives of the same class will perform similarly. By the same "class" we mean the same speed, the same interface, and similar prices. Sure, there will be differences, but for most mainstream users they probably won't be significant enough to worry about.

That said, the following is a list of the main things to consider when selecting a hard drive.

form factor (size)
internal or external drive
interface
internal performance
internal data transfer rate
positioning performance
capacity
manufacturers
reliability/warranty
Form Factor
Start off by selecting a form factor. A drive's form factor is its physical size. Currently the best form factor for a desktop PC is 3.5 inches. These drives are significantly faster than 5.25" and 2.5" drives. However, 2.5" drives are commonly used for notebook computers because they weigh less.

Internal or External Drive
You'll need to choose whether you want an internal drive or an external drive. To connect an internal drive to your system you'll need an open internal drive bay. Often cases have a special hard drive "cage" with slots for a couple hard drives should you want more than one. The cage can usually be removed from the case entirely to make it easier to mount the drive. If you don't have an available drive bay, you'll need to purchase an external hard drive, which will plug into a port on your computer and sit on your desk. External drives usually cost about $100 more than equivalent internal drives. In addition, there isn't as much of a variety in external hard drives as internal.

Interface
As mentioned earlier, when you "Save" a document it travels from the computer's main memory to the hard drive via an interface. You can think of an interface as a freeway for data. There are different interfaces for external drives than there are for internal drives.

External Drives

The two most common types of interfaces for external hard drives are USB 1.1 (universal serial bus) and IEEE 1394 (also referred to as FireWire or iLink). FireWire is a higher performance interface than USB, having a maximum bandwidth of 50MB/sec verses USB's 1.5MB/sec. Bandwidth refers to the amount of data that can flow across the interface in one second. So, in almost all cases FireWire hard drives will be better performers than USB hard drives.

However, a new edition of USB has recently arrived on the market--USB 2.0. USB 2.0 actually offers a slightly higher bandwdth than FireWire, and therefore is also a viable interface option for external hard drives.
USB 1.1, USB 2.0, and FireWire are plug-and-play interfaces, which means it should be simple to connect your drive; you won't even need to turn your computer off. In addition, if you find that your computer is not equipped with the interface you need, you can purchase an adapter card that plugs into a PCI slot in your motherboard. Such adapter cards currently range in price from about $25-$80.

Internal Drives
When it comes to internal hard drives, two completely different types of interfaces are predominant: EIDE (Enhanced Integrated Drive Electronics), and SCSI (Small Computer Systems Interface). SCSI is the more technologically advanced, and expensive of the two. EIDE is the more mainstream.

If you are a mainstream computer user, for instance you primarily use your computer for basic office work, browsing the Internet, and games, EIDE is probably going to be the better option. For one thing, most motherboards are already equipped to support EIDE drives*. If you want to hook up a SCSI drive, however, you'll most likely have to purchase a SCSI host adapter (often referred to as a SCSI controller card, though that's not quite accurate), which can cost quite a bit depending on the quality.

*Note: Some older motherboards may only support IDE--an earlier, less advanced version of EIDE.