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How do Hard Drives become Fragmented?

Updated on December 3, 2013
Defragmenting in Windows 7
Defragmenting in Windows 7

A quick explanation on how files get fragmented and how to defragment them


The Second law of thermodynamics states that in an isolated system, the entropy of that system will either increase or decrease. Entropy in other words is chaos or the opposite of order. This law is also applicable in Hard Drives.

Data in a hard drive or any storage device are stored in blocks. Each block has a segment of data which forms the actual file. When you first write data into a hard drive, data blocks are stored contiguously – they are beside each other. For example:

A A A A B B B B C C C C D D E E E E E F F F _ _ _ _ _ _ _ _

Let each letter represent a block of a file. This is how data looks like when you first write in your hard drive. Now let’s say you erase file D, here is how it will look like:

A A A A B B B B C C C C _ _ E E E E E F F F _ _ _ _ _ _ _ _

Now let’s say you write into your drive file G which is six letters long. Sometimes, your computer will chose to write it in contiguous space. If it does, it will look like this:

A A A A B B B B C C C C _ _ E E E E E F F F G G G G G G _ _ _

In Windows XP, when you use disk defragmenter, you’ll see a bar with a lot of colors in it. White is free space and blue is contiguous files. Sometimes white space appears between blue space and what we have shown above is the reason how that happens.

Now, let’s say you erase F and B then write another six letter file called H. Here is how data in that hard drive will look like:

A A A A H H H H C C C C H H E E E E E _ _ _ G G G G G G _ _ _

Erase C then Write a five letter file called I.

A A A A H H H H I I I I H H E E E E E I _ _ G G G G G G _ _ _

Erase E then put back B and C.

A A A A H H H H I I I I H H B B B B C I C C C G G G G G G _ _ _

After a few write and erase cycles we now have three fragmented files. That’s how it happens in your hard drive. And Fragmentation slows down your computer’s ability to read data from your hard drive. Reading from a fragmented drive is like reading from a book that had its pages jumbled up. Once your drive is fragmented you need to use a built-in maintenance tool in windows called disk defragmenter.

To open up disk defragmenter, open My Computer then right click on a drive then choose properties then click tools then defragment now. In Windows 7, just type defragment in the start menu. Defragmenting takes a lot of time especially for drives that has a lot of files in it that are fragmented.

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