What Is "Defragging," and Do I Need to Do It to My Computer?
Most hard drives have spinning platters, with data stored in different places around that platter. When your computer writes data to your drive, it does so in "blocks" that are ordered sequentially from one side of the drive's platter to the other. Fragmentation happens when those files get split between blocks that are far away from each other. The hard drive then takes longer to read that file because the read head has to "visit" multiple spots on the platter. Defragmentation puts those blocks back in sequential order, so your drive head doesn't have to run around the entire platter to read a single file.
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Fragmentation doesn't cause your computer to slow down as much as it used to—at least not until it's very fragmented—but the simple answer is yes, you should still defragment your computer. However, your computer may already do it automatically. Here's what you need to know.
If you have a solid-state drive (SSD) in your computer, you do not need to defragment it. Solid-state drives, unlike regular hard drives, don't use a spinning platter to store data, and it doesn't take any extra time to read from different parts of the drive. So, defragmentation won't offer any performance increases (though SSDs do require their own maintenance).