Do You Need to Defragment a Mac's Hard Drive?

Apple supplies a handy application for working with hard drives called Disk Utility, but it lacks a tool for defragmenting the drives connected to your Mac. The reason: A Mac running any version of OS X later than 10.2 or macOS does not need to be defragmented. OS X and macOS have their own built-in safeguards that prevent files from becoming fragmented in the first place.

Anti-fragmenting is built in

The Mac's HFS+ file system tries not to use recently freed file space on a disk. Instead, it looks for large free areas already present on the drive, thereby avoiding fragmenting files just to fit them into available space.

The Mac OS dynamically gathers groups of small files and combines them into large areas on your disk automatically. The process of writing the files to a new larger location defragments all of the files in the group.

OS X and macOS implemented Hot File Adaptive Clustering, which monitors frequently accessed files that do not get changed (read-only), and then moves these often-accessed files to a special hot zone on the startup drive. In the process of moving these files, the operating system defragments them and stores them in the area of the drive that has the fastest access.

When you open a file, the Mac checks to see if it is highly fragmented (more than 8 fragments). If it is, the operating system will automatically defragment the file.

The result of all these safeguards is that a modern Mac rarely, if ever, needs to have its disk space defragmented. The only real exception to this is when your hard drive has less than 10% free space. At that point, the operating system is unable to perform its automatic defragmentation routines, and you should consider either removing files or expanding your disk storage size.

Is There Any Reason Not to Defragment My Mac's Drive?

Some types of tasks can benefit from defragmented drives—specifically, real-time or near-real-time data acquisition and manipulation. Think video or audio recording and editing, complex scientific data acquisition, or working with time-sensitive data.

This applies only to standard hard drives. If you’re using an SSD, or a Fusion drive, its data should never be defragmented. Doing so can lead to writing amplification, a common cause of premature failure of the SSD. SSDs have a finite number of writes that can be performed. You can think of it as the memory location within the SSD becoming brittle with age. Each write to a memory location increases the age of the cell.

Because flash-based storage requires the memory locations to be erased before new data can be written to them, the process of defragging an SSD can lead to multiple write cycles, causing excessive wear on the SSD.

Will Defragmenting Harm My Drive?

As we mentioned, defragmenting an SSD or any flash-based storage device (this includes Fusion-based drives that use a small SSD/flash device along with a standard hard drive) can lead to premature failure by increasing the amount of wear (writing and reading of storage cells). In the case of a conventional hard drive, which uses a mechanical rotating platter, there's no significant risk of damage in performing a defrag. The only negative is in the time it takes to perform the defragmentation.

What If I Decide That I Do Indeed Need to Defragment?

Third-party utilities such Drive Genius 4 can defragment your Mac's drives. It also includes the ability to monitor drive health and repair most drive problems.