An alternative to iDefrag is Drive Genius 4, the software that Apple employees use at the Genius Bar. While it's more costly than iDefrag, coming in at $100, Drive Genius 4 comes with a tool that allows you to create a secondary drive so you can defrag & repair your main hard drive. Don't worry, that's not all you get for $100 (around £64.55) though - it comes with a host of different features to free up space on your Mac and protect your HDD.
DO NOT defrag a Mac's SSD
If you're using a newer Mac that comes with an SSD (Solid State Drive), there's no point in wasting your time defragging as they already have a built in maintenance process called TRIM. In fact, it's actually dangerous to attempt to defrag a SSD, which can reduce its lifespan.
In any case, SSDs operate in a different way to a traditional HDD, allowing quicker access with fewer issues making them the preferred option for modern systems.
Alternatives to defragging: Repairing Disk Permissions
There are alternatives to defragging if you have issues with your Mac's general speed.
The most popular piece of advice that Mac users will give is to "Repair Disk Permissions" using Disk Utility, software that's baked directly into OS X and allows HDD management and repair. In reality, it only fixes file permissions that were installed as part of an Apple-originated installer package - so those that came pre-installed with your Mac and files downloaded from the Mac App Store.
Permissions control which users and system processes have access to certain files, with all permissions collected in "Bill of Materials" database files (.bom) in your OS X System. During the normal use of a Mac, it's possible that some file permissions will change from what was originally set. When this happens, things generally go wrong - a program might give your user account read-only access to your home folder and restrict access to your files, for example.
People can often read this as something that a disk defrag could fix when in reality, all you need to do is repair your disk's permissions - a simple (and risk-free) process.
When you repair your permissions, your Mac looks at the .bom files and checks with the files on your system, making sure that the permissions match. If they don't, the software will automatically change the file permissions so that they match those originally set in the .bom files, hopefully fixing any issues that you had.
To repair your disk permissions, simply open up the Disk Utility, select your hard drive and click Verify Disk Permissions. The Disk Utility will then scan your Mac and highlight any permissions that are inconsistent with their related .bom files. Once this process is finished, you can click Repair Disk Permissions to fix any issues with permissions - a process that shouldn't take more than a few minutes, depending on your Mac's general health.
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