To understand why fragmentation occurs, we must first gain a brief understanding of how a computer stores information. Think of your hard drive as a large grid full of thousands of different blocks. Each one of these blocks has a number assigned to it by your file system (known as an address), for organizational purposes.
When your computer stores information on your hard drive, it requests the required amount of free blocks from your file system, and the file system returns the address to each of these blocks. In an ideal world, these blocks would always be right next to each other, and read at the same time you want to retrieve that file.
Sometimes, however, the file may be stored at several different locations scattered throughout your file system. Therefore, instead of reading a set of blocks in order, it will read a few blocks, then have to look for the next set of blocks, read them, and look for the next set.
The same thing applies to
writing files as well.
Because your hard drive can
only read one block at a
time, you can probably
understand why this would
slow down your disc read and
write speeds, thus slowing
down your PC. When this
happens, it is time for a
defrag.
