Disk Defragmenter

Whenever your computer writes a new file to the hard disc, the area on the disc to which the file is written is decided by a placement policy. This basically tries to figure out the best place to put the file. For example, the OS may decide that to make things quick, it'll chuck the file into the first piece of free space on your drive that is large enough to hold it; that's called "first fit". Alternatively, it might look for an empty space that most closely matches the size of the file you're writing to the disc. That policy is called "best fit". There are a few different methods.

A problem inevitably arises when your drive begins to fill up to the point where there are not many free areas large enough to hold a whole file. In this case, the OS "fragments" the file (which basically means it chops it up into as many pieces as necessary to make all the data fit into whichever gaps are left). If you have large files like a DVD rip, the ideal way for it to be stored would be as one big piece, so that the heads in the hard disc that read data don't have to move around. In reality though, you're unlikely to have an area of free space on your disc large enough to hold a huge data file as one chunk. As a result, the heads in the hard disc that read data will have to move to many different locations on the disc surface to read all parts of the file (this is called seeking). The more often the heads have to move, the more time is wasted seeking information rather than reading it. This slows overall system performance and in extreme cases you might experience video playback that skips.

The defragmenter utilities available in Windows and as third-party software all basically try to collect the fragments of files and rearrange them on your hard disc in order to improve overall performance and minimise the amount of seeking required to access files. This usually involves trying to bunch the pieces back together in some way, and moving files to locations on the disc that are more quickly accessible (near the outer edges of the drive surfaces). Sorry about the long explanation.

Jam it back in, in the dark.