Hard disc drives store data in several areas called sectors. In each sector stores a certain amount of data. Data is read from the hard disc drive from one sector to the other. When you delete files on your hard disk, the data structure is changed, and the disk becomes fragmented, meaning data takes longer to be read and processed.
Defragmentation optimize this process, and this optimization has two effects: one is that the hard disc drive will have more space, because "holes" created when you delete files, will be freed up and the second effect is that the arm that reads the data from the hard disk drive, won't have to move back-and-forth so much, which will improve read speed.
The short answer is: Yes, defrag works!
If it's been a long time since you last performed a defragmentation, perhaps several months or years, and you have also installed a lot on your PC and delete lots of files, there is little doubt that you will see an improvement in both speed and space freed up.
However, if you conduct defragmentation on a brand new PC, you will not notice anything. You will not notice anything if you run defragmentation frequently, such as once a week. Defragmentation is, in other words certainly useful, but you only need to perform it every month or 2 to keep your PC in tiptop condition.
Before you perform a defragmentation you should perform the following:
Though it's extremely unlikely a disc defragmentation will cause any problems, it is always advisable to back up important files.
You may want to remove unwanted files and programs before performing a disc defragmentation. We recommend CCleaner for this task.
Create a recovery point just in case if something unforeseen happens during defragmentation.
It is advisable to close all programs before proceeding with defragmentation, so the process is not disturbed. You may also find if you perform a defragmentation while using your computer, that it may become sluggish and unresponsive.
Is there any point in performing a defragmentation on an SSD device? If you google this, you will hardly find a single website that recommends defragmenting a solid-state drive.
Most sites say that this is unnecessary, and some even say that it can be harmful to perform this task.
Scott Hanselman from Microsoft, however, believes defragmentation is not harmful for SSDs and believes that they can even be positive for the performance, at least if you use Windows. The reason for this, says Hanselman, is that it is the file system that is the determining factor, and not the medium. Read more on the blog of Scott Hanselman.