This is one of those questions that gets my most common and my most favorite answer: “It depends.”
It depends on the drive. It depends on your computer. It depends on how you use your computer.
So, how can we narrow this down?
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Defragging your C: drive
Frankly, defragging any drive other than the system drive (i.e. your C: drive) is typically not important. It’s probably not something that you’ll want to do often.
Why? The C: drive usually does the most work. That’s where Windows picks up its files from; it’s where all of your programs are installed; it’s where Windows is reading and writing to the registry.
You definitely want to make sure that the C: drive is defragged regularly. Personally, I kind of like the settings in Windows 7 to defrag all of the drives that are installed on the system once a week.
When it comes to other drives, like your D: drive or even external drives (if you’ve got them), it gets a little weirder.
If the drives are used really lightly, I don’t think you’ll find that defragging is really going to buy you that much. It certainly doesn’t hurt to do it, other than the time it takes, but defragging a lightly used drive is just not going to have that much performance impact. And that’s really all that defragging is about – improving performance and making the disk access faster.
When to defrag other drives
However, if you are doing things with your other drives like moving your paging file, storing your temporary files, or reading and writing documents or to a stored database, then defragging could make a lot of sense. You should probably defrag on the same once-a-week schedule with your C: drive.
In either case, there’s no harm in defragging your D: drive. In fact, if you’re running Windows 7 and the automatic defrag is set to happen, chances are your D: drive is already being defragged along with the C once a week.
Exception to the rule
Why? Well, you’re not going to get much if any of a performance boost (there’s some controversy around this, but supposedly defragging these drives is just not worth it).
Because defragging reads, and particularly writes, so much to the drive, it will actually cause the SSD or USB flash drive to wear out much faster.
So, in short, if you have a heavily used drive, make sure that it’s getting defragged on a regular basis. If it’s not getting used that heavily and it’s idle most of the time, there’s no harm in defragging it, but I wouldn’t take any additional steps or go out of my way to make sure that it was being defragged on any kind of a regular basis.