HARD Drives

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• What are the differences between hard drives?
• Different types? Hard drive speed and capacity?
• Hard drive uses and storage?
• What Size Should I Buy?

The Bigger the Better—Usually

The rule of thumb for hard drives has always been that you can never have too much storage, so you should buy as much as you can afford. In general, that's still true, even though you can now buy 100GB or more of storage for well under $200.
Buying lots of capacity is smart because data files tend to grow to fill the available space. Careful users try not to clutter up their disks with unnecessary files, but more and more software fills up the space for you, on the grounds that with today's hard drives you can lose 1GB or so without noticing it. As the trend continues, however, it's likely that a drive that seems overly generous today will run out of room at some point in its lifetime. The bigger the drive, the less likely that will happen—or, at least, the longer it will take. There are some limitations on capacity. The obvious one is that higher-capacity drives cost more than low capacity drives, even though the price per gigabyte goes down. Another issue is that for any given budget, you may decide to go with a slightly lower capacity for the sake of better performance. Note too that if you have an older computer, you may run up against BIOS limitations for how large a capacity the computer can handle.
What Specs Should I Look For?

Take Specs With a Grain of Salt:
When you look at drive specifications, keep in mind that there are only two easy cases for predicting performance based on specifications. If one drive outdoes another on every single performance specification, it's a safe bet that it will offer better performance. Similarly, if two drives match in all but one specification, the one that does better in that one specification will be the better performer. Beyond that, judging performance by specifications is almost impossible, because the performance depends on several factors and the balance between them. Still, there are some basic indicators that can help you make rough comparisons.
The two specifications you should care most about are two that you'll rarely see advertised: average sustained transfer rate and average access time. All the performance specifications you see most often—burst transfer rate, internal transfer rate, rotation speed, average seek time, latency, and density of data on the disk—matter only because they affect these two key specifications.
Transfer Rate

There are several kinds of transfer rate, and they are not interchangeable, so be careful when comparing drives.

• Internal transfer rate measures the speed that data can move between the drive head and the drive platter. This is usually the limiting factor in transferring data to and from the drive.
• Burst transfer rate is the maximum speed of the interface. This is always faster than the internal transfer rate. The only time a drive will transfer data at the burst rate is when moving data to and from the memory buffer in the drive
• Sustained transfer rate measures the speed that data can move between the drive and the computer for sustained periods of time.

The transfer rate you'll see most often is the burst transfer rate, because it's the most impressive number of the three. But the number that matters most is the sustained transfer rate, which essentially tells you how quickly large amounts of data can move to and from the drive. Most of the time, the drive will have to read from or write to the disk platter itself, which means the sustained transfer rate depends primarily on how quickly the drive can move data between the drive head and the disk. The sustained transfer rate is equal to the internal transfer rate minus whatever overhead the drive needs for transfer operations.
You'll sometimes see specifications that give a minimum, maximum, and average sustained transfer rate. The difference comes from the fact that the outer tracks of a disk platter have more data, so more data passes under the drive head during a single rotation when reading from the outside tracks than when reading from the inside tracks. The maximum sustained transfer rate is the transfer rate for the outer tracks. The minimum is the transfer rate for the inner tracks. The average is the average over the entire disk. Other factors that affect transfer rate include the density of the data on the disk and the speed of rotation.
Average Access vs. Average Seek (and Latency)

Access time: tells you how long, on average, it takes the drive head to reach a designated spot on the disk. The faster the access time, the faster the drive will find a collection of small pieces of data, like a selection of records when you search a database or suggested alternatives for a misspelled word.
Access time consists of two components: seek and latency. Seek time is the amount of time it takes the drive head to move along the diameter of the disk to reach the right track. Latency is the amount of time it takes for the disk to rotate under the drive head to bring the right spot on the track under the head. The faster the rotation rate, the lower the latency. When you look at a drive's specifications, be sure you know whether you're looking at average seek time or average access time (sometimes seek is mislabeled as access time), and be careful not to compare the seek time from one drive to the access time of another.

Rotational Speed: The rotation rate tells you how fast the disk spins under the drive head. Typical rates for mainstream desktop drives are between 7,200 rpm and 10,000 rpm. High-performance drives often offer 15,000 rpm, and faster.
All other things being equal, the faster the rotation rate, the more data will move under the head in a given amount of time, the more data the head will read, and the higher the sustained throughput will be. However, rotation rates don't compare as neatly as they might, because all other things are not usually equal.
Rotation rate also affects average access time, because one of the two components of access time—latency—is determined by the amount of time it takes the disk to rotate the right spot on a track underneath the drive head after the drive head has reached the right track. The faster the rotation rate, the lower the latency, and the faster the access time.
Density

The density of data along a track, or linear density, obviously affects the capacity of a drive, because denser data means more data. Less obvious is that linear density affects performance. A drive with a higher linear density than an otherwise identical drive will have more data passing under the drive head with each rotation. That means the drive head can read more data in a given amount of time, and the drive will have a faster transfer rate. If one drive has a high enough density compared to another, it can have more data passing under the head even with a slower rotation rate; therefore, a high-density drive with a low rotation rate can have a faster transfer rate than a lower-density drive with a faster rotation rate. You'll rarely see the linear density specification for a drive, but you should keep this fact in mind when comparing rotation rates.
Interface

Internal drives for desktop systems come in many choices: IDE, SCS and SATAI. They all come in several variations. One of the key differences between one IDE or SCSI version and another is the transfer rate, with the interface transfer rate serving as the maximum rate, or burst rate, for the drive. Although adding a faster interface to a slow drive won't necessarily improve performance, a slow interface can slow performance down for a fast drive. Every time drive design improves to the point where the transfer rate at the drive head is in danger of catching up with interface transfer rate, the industry comes up with a newer, faster version of each interface, which is why there are several versions of hard drives.

CapacitY: Capacity is obviously an important issue in picking a drive. In addition to getting enough capacity, however, you need to make sure your system can handle the drive capacity you get. The BIOS’s in some older systems have a problem with any drive larger than 80GB. Some systems will recognize only 60GB of a larger drive. Other systems have similar capacity ceilings at 100GB, or some higher level. It's best to check with your system vendor beforehand to find out if there's a problem with the drive capacity you want. If there is, you may be able to fix the problem with a flash upgrade if your system uses a flash BIOS or even a simple download .exe file.

Form Factor: is just a way of describing how large a drive is and telling you what size drive bay you need. The vast majority of internal hard drives for the desktop are 3.5 inches. A few exceptions, typically including the largest capacity drives available, are half-height 3.5-inch drives. When you choose a drive, make sure you have an appropriate-size bay available for whatever drive you get. A 3.5-inch drive can fit in either a 3.5-inch or half-height 5.25-inch bay, but you may have to buy a separate mounting kit to install it in a 5.25-inch bay.
Related to form factor is the issue of internal versus external drives. The rule for choosing between the two is simple: Unless you have a good reason for getting an external drive—like not having a drive bay available—get an internal drive. It will save room on your desktop and typically save you $100 or more, because you won't have to pay for a external case. Consider an external drive if you don't have a drive bay available, want to move the drive between systems, or you want the convenience of plugging in the new drive without opening the case.
Source: http://www.ivitex.com/faq2.htm           

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