Hard Disk Drive

Hard Disk Drive

(Also known as Hard Drive or HDD)

        A hard drive is the standard device for storing information on a computer. It stores data on disks known as platters that contain circular tracks with a magnetic surface. Modern hard drives use several platters spinning at high speeds to access (Read) and store (Write) data, this is done by an arm, which positions read/write heads in the right tracks to modify data. Think of it as a very advanced record player. Hard drives are, in general, known for their high storage capacities, low cost, and good over-all read and write speeds. Compared to solid state drives, they are large, consume more power, make more noise (SSDs are completely silent), and are slower than recent SSDs.

        There are several factors that contribute to the performance of a hard drive, the most important is the spindle speed followed by platter diameter, areal density, and buffer size. this allows for more data to be transferred in the same amount of time. Higher speeds also allow for lower access times, since less time is spent waiting for the right section to be reached.

Form Factor

        There are several common sizes for hard drives. A 3.5 inch form factor, (Usually 4 in by 1 in by 5.75 in) is the standard size for desktops is. This standard fits into most any hard drive bay in common desktop cases. The other common size is 2.5 inch hard drives (Usually 2.75 in by 0.374 in by 3.945 in). These are seen in most laptops  occasionally slightly higher hard drives, around 12 mm, are used for high capacity hard drives. There are also smaller 1.8 inch hard drives for ultra portable computers, but these are being replaced by SSDs now days.

        Note- You might have noticed that a 3.5 or 2.5 inch hard drive does not actually have dimensions of 3.5 or 2.5 inches, this, refers to platter diameter, not actual physical size.

Storage Capacity

        Modern hard drives have a wide range of storage capacities. Simply, storage size is determined by the storage capacity per platter, and how many platters there are. The key factor is areal density, which allows for platters with higher capacities to be made, increased areal density allows for hard drives to be faster as well, more on that later. In general hard drives have between 1 and 5 platters, more platters allow for more storage, but hard drives with more platters consume more power, make more noise, and tend to have a higher risk of failure. Typical hard drive sizes range from 120 gigabytes to 2 terabytes, for desktop hard drives, and 60 gigabytes to 500 gigabytes for laptop hard drives. Often times a hard drive may have a lower capacity than one would get if they added up the density of the platters, for instance a 640 GB hard drive might have 3 platters capable of storing nearly 250 GB each, manufacturers often don't use all parts of a platter, so that the fastest sections are used (Near the outer edge) and the inner sections are disabled, this allows for higher performance, as well as being able to fit into a ideal size range to sell a hard drive.
       

Spindle Speed

        Spindle speed is measured in rpm, rotations per minute, the higher the speed the better the performance of the hard drive. Higher speeds are important as this allows for more data to be transferred in the same amount of time. Higher speeds also allow for lower access times, since less time is spent waiting for the right section to be reached. Spindle speed for desktop computers is usually 7,200 rpm but slower hard drives are offered at 5,400 rpm. There are also high-end 10,000 rpm models, most notably the Western Digital Raptor series, which offer the best performance possible, but have significantly lower capacities (300 gigabytes is the max), and cost more than a comparably sized 7200 rpm hard drives. On the laptop side of things, most hard drives are 5,400 rpm. However, there are also slower 4,200 rpm hard drives and also some 7,200 rpm hard drives, available for a price premium. It is also important to note that higher spindle speeds result in higher power consumption.
      

Platter Diameter

        Platter Diameter is very important for hard drive performance. Since the outside of the hard drive is moving much faster than the inside, the larger the diameter of the platter, the faster a hard drive will be. For instance, a 3.5 inch 7,200 rpm hard drive will perform better than a 2.5 inch 7,200 rpm hard drive. This is one of the main reasons for performance decreases in laptop hard drives compared to desktop ones.

Density

        The density of a hard drive is often difficult to find in its most effective form, gigabits per square inch, however general assumptions can be made about a hard drives density and how it affects performance. The best gauge of density is a hard drives age, newer models of hard drives will have higher densities than older ones, and therefore, perform better since data is packed together more tightly and takes less time to cover. The corollary of this is a hard drives size, a hard drive with a larger storage capacity will likely have a higher density. However, this is where platters become important again, as two hard drives may have the same storage capacity, say 1 TB, but one may have 5 platters of 200 GB each, and the other have 3 platters of 333 GB each. The second one has a higher density and thus will have better performance.

Buffer Size

(Also known as Disk Cache)

        Modern hard drives contain embedded memory for storing recently accessed data, as well as dealing with commands sent to the hard drive. Buffer size varies from 8 mb up to 32 mb in curent hard drives. Large buffer sizes result in a slight improvement in performance although it is negligable when comparing similar hard drives. It should still be considered however.


source:knol.google.com/k/hard-disk-drive-hdd#