Prabaharan(classmate/no-eligible-but-is-a-prefect): "Defragment is you know, delete the files inside the hard disk to make it go faster."[he also said that he reformatted his 350GB hard drive on a Pentium 4 computer + install Windows XP in 45 minutes all through BIOS. WTF!?]
About half of my class said something like that when I put this question on a debate. These people(including you) should read this: http://computer.howstuffworks.com/question548.htm
If you don't understand it, I'll do the explaination to you. I'll start by explaining what are and how hard disks work.
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Hard disks are just a metal box with a few layers of special discs called hard platters each with an arm that acts as a data reader and writer. The arm moves about a side of the disc to go to some parts of the drive.
Now, here is a diagram of hard platter. Data is stored on the surface of a platter in sectors and tracks. Tracks are concentric circles, and sectors are pie-shaped wedges on a track.A typical track is shown in yellow; a typical sector is shown in blue. A sector contains a fixed number of bytes -- for example, 256 or 512. Either at the drive or the operation system level, sectors are often grouped together into clusters.
So let's imagine that you install a new application onto an empty hard disk. Because the disk is empty, the computer can store the files of the application into sequential sectors on sequential rings. This is an efficient way to place data on a hard disk.
As you use a disk, however, this efficient technique becomes harder for a disk. What happens is that the disk fills up. Then you would erase files to reclaim space. These files that you delete are scattered all over the surface of the disk. When you load a new application or a large file onto the disk, it ends up being stored in hundreds or thousands of these scattered pockets of space. Now when the computer tries to load the scattered pieces, the disk's arm(which is very slow) has to move all over the surface and it takes forever.
The idea behind the disk defragmenter is to move all the files around so that every file is stored on sequential sectors on sequential rings of the disk. In addition, a good defragmenter may also try to optimize things even more, for example by placing all applications "close" to the operating system on the disk to minimize movement when an application loads. When done well on older disks, defragmenting can significantly increase the speed of file loading. On a new disk that has never filled up or had any significant number of file deletions, it will have almost no effect because everything is stored sequentially already.
As you might imagine, the process of indivdually picking up and moving thousands of files on a relatively slow hard disk is not a quick process -- it normally takes hours.Sources: HowStuffWorks - http://computer.howstuffworks.com/question548.htm
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