The purpose of a storage device is to allow the writing and reading of data organized into collections called files. Hard drives are very similar to floppy drives. A hard drive, as its name implies, stores data on a rigid magnetized disk or set of disks. Hard disks pack data much more densely than floppy disks.
Because a single hard drive unit may contain multiple disks, the storage capacity is much greater. Where a single floppy disk may contain several megabytes of data, a hard drive can store 100 gigabytes of data or more, which means one hard drive is equivalent to tens of thousands of floppy disks. Hard drives are also much faster at writing and retrieving data.
To achieve this higher density and performance, hard drives spin thousands of times per minute, and use a faster and more precise mechanism for controlling the arm the heads ride on. One measure of a particular hard drive's performance is seek time, which is the length of time needed to move the heads over a specified track. Current hard drives have seek times under ten milliseconds, or put another way, they can find a track in less than 1/100 of a second.
Although technically a "hard disk" would be a disk inside the hard drive, in common use "hard disk" and "hard drive" are used interchangeably to refer to the entire device, not a particular disk inside the device. This is probably because hard drives are sealed. The media is not only not removable, but also can't be seen without opening the drive casing. Hard drives are sealed because the data is so precisely aligned than any dust that fell on a disk could ruin the data stored there.
Hard drives are the central storage device on computers today, and their capacity seems to double every couple of years. One problem larger drives have exacerbated is fragmentation, which is the state where files have been split up into small pieces across the disk. When a hard drive is new, the disk is one big block of unused storage, and files can be stored contiguously on the disk, perhaps all on the same track. Over time, as files are stored and erased, they split up the leftover space into smaller and smaller chunks.
Eventually individual files are too big for any one chunk and have to be split up and spread across multiple tracks. Although seek times for drives are very fast, a file can still be read much faster when it is on the same track. Having files spread all over the disk is analogous to a messy desk: Although you can find what you need eventually, you have to sift through the piles to find it.
Fragmentation is one reason why computers that have been in use for a long time seem to have slowed down from when they were bought. The closer a hard drive gets to capacity, the more fragmentation occurs. A program called a defragmenter carefully shuffles the file fragments around to get them all contiguous again, improving performance.
An optical drive reads data using a laser and light sensor. Bits are represented on an optical disc by placing bumps on the surface. The laser light reflects differently off the bumps and smooth parts of the disc to produce the binary 1 and 0 signals. The first consumer use of optical drives was not for personal computers but for music. The availability of compact discs, or CDs, enabled consumers to purchase music in digital form, rather than analog form as on vinyl records. Although the CD was eventually co-opted by the computer industry, its specifications were dictated by the music industry.
A music CD can hold about seventy-four minutes of music. This time was selected so it would hold as much as any single vinyl record. Because of the sample rate and bit rate chosen by the industry, this works out to a data capacity of about 780 megabytes.
Unlike floppy and hard disks, a CD doesn't have concentric tracks. It contains one long spiral track, just like a vinyl record. A single track is used because a music CD needs to continuously read the disc to play the music. If it had to jump from one track to another, small gaps would appear in the music playback. The single track causes trouble when applied to computers, though, because the CD is used as a random access device. Finding the right spot to start reading on a CD is much slower than finding a track on a floppy or hard disk. It can take about ten times as long.
If you've ever been on a merry-go-round, you know that you go faster on the edge than you do toward the middle. To counter this effect, a CD player spins the disc slower as the laser moves to the outer part of the disc. Again, this is done because the data needs to be read and turned into music at a constant rate.
An optical disc used for storing computer data is called a CD-ROM. A CD reader in a computer system is known as a CD-ROM drive. In both cases the ROM stands for read-only memory, which means storage data that can be read but not altered. CD-ROM drives can read and play music CDs but are mainly used to install software on CD-ROMs. Installation refers to a process where software is copied onto the hard drive and registration steps are taken to connect the software to the operating system, including making the new software appear in the operating system's "list of programs."
Early CD-ROM drives read data at the same speed as a music CD player, which by computer standards is very slow. Logically, this meant that reading the entire disc would take seventy-four minutes! Current CD-ROM drives can read about fifty times faster than a CD player and are able to read the outer part of the disc at about the same speed as the inner part.
While CDs and CD-ROMs are manufactured with the bumps in place, another technology allows users to make discs with their own computers. A CD-RW drive, or computer disc read/write drive, can read discs and create discs using specially made blanks. The blank discs are quite different from ordinary CDs.
They contain a special dye that is initially clear but darkens when heated. A CD-RW contains a more powerful laser that is used to heat up the dye in different spots to make reflecting and nonreflecting areas that can be read like a regular CD. Because of the heat involved in the dye process, creating a CD in this way is known as burning.
Some blanks are called CD-Ws (W for write-only) and can be used only once because after the dye has darkened, there's no way to make it clear again. Other blanks are called CD-RWs (RW for read/write), which use a substance that is heated to one of two temperatures. Heated to one temperature, that spot on the CD is reflective when it cools; heated to another, it is opaque.
The DVD, a more recent invention, works along the same principles. A DVD is an optical disc of much greater capacity than a CD-ROM, and a DVD drive is a device in a computer system for reading DVDs. The name DVD originally stood for Digital Video Disc because it was developed to store movies digitally, but later the name was changed to Digital Versatile Disc because any kind of data can be stored on a DVD, just like a CD.
A basic DVD can hold about 4.7 gigabytes of data, which is about six times as much as a CD-ROM. Some DVDs have two layers of data. The bottom layer is like that on a CD-ROM, a shiny surface with smooth spots and bumps. Above that is a semitransparent layer that's more like a CD-RW, with lighter and darker spots. Having both layers allows the capacity to almost double to around 8 gigabytes. You will sometimes see a 17 gigabyte capacity listed for DVDs—for a double-sided DVD that uses two layers on each side.
Because most DVD readers can only read one side, though, the user has to eject the DVD, flip it over, and put it back in the reader. From a user's point of view having a double-sided DVD isn't better than having two single-sided DVDs. As with CDs, blank DVDs and DVD burners are also available and work along the same principles.
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