Guides

Practical, tested guides to macOS storage.

Eleven guides to macOS storage: what the System Data label actually contains, where the space sits, and the Terminal commands that measure it correctly. Every figure comes from one 512 GB MacBook Pro taken from 20 GB free to 215 GB free in September 2026, and every command was run before it was published.

Dustloft is a free, open-source macOS app that shows what is hidden inside the storage macOS labels “System Data” and reclaims it. It runs on macOS 13 or later as a universal binary, is MIT licensed with no account or subscription, and lists every item with its full path and real size before anything is removed.

• Why is System Data so large on your Mac?

  System Data is not one thing. It is every file macOS cannot file elsewhere. What is actually inside it, with real sizes from a 512 GB MacBook Pro.

• How to clear System Data on a Mac without breaking anything

  A step-by-step guide to reclaiming System Data on macOS: the exact Terminal commands, what each one does, and which things you must never delete.

• Free CleanMyMac alternatives in 2026, compared honestly

  CleanMyMac costs money and every job it does has a free equivalent. The free tools that actually work, what each is good at, and where each falls short.

• Docker.raw is taking up hundreds of GB on your Mac — here is why

  Docker.raw can report 494 GB while occupying 9 GB, or quietly consume 300 GB of real disk. Sparse files explain both. How to measure and reclaim it.

• How to find what is taking up space on your Mac using Terminal

  The du, df and mdfind commands that actually work on macOS, including the common mistake that silently hides tens of gigabytes of hidden folders.

• What is purgeable space on a Mac, and can you actually free it?

  Purgeable space is disk macOS has promised itself it can reclaim. No third-party cleaner can free it. What it contains and what genuinely moves it.

• Is it safe to delete ~/Library/Caches? Mostly yes, and mostly pointless

  Emptying ~/Library/Caches is the most repeated Mac advice and one of the least effective. The gigabytes are next door, in Application Support.

• WhatsApp and Messages media: the 92 GB hiding in Group Containers

  Messaging apps keep every photo and video you have received in Group Containers, hidden from Finder. On one Mac that was 92 GB, and it is permanent.

• node_modules and build output: the 66 GB most developers never look at

  Dependency folders reached 46 GB and stale build output 20 GB on one Mac. How to measure them, and the one deletion that quietly breaks a library.

• Xcode leftovers: DerivedData, simulators and runtimes you can safely remove

  Xcode keeps build indexes, simulators and runtimes long after you need them. On one Mac, 13 GB of orphaned runtimes were still present.

• Git repositories are taking up space. Repack them, do not delete them

  A .git directory can reach several gigabytes and is the one irreplaceable folder on a developer machine. How to shrink one safely with git gc.

How these numbers were measured

Three capacities for the same disk appear across these guides, and they are the same disk measured three ways. 512 GB is the marketing capacity, the figure the machine is sold as. 494 GB is what is left once the drive is formatted, which is why a Docker disk image reporting 494 GB was reporting the size of the whole drive. 460 GiB is the same 494 GB counted in binary units rather than decimal ones, and it is the figure df -h /System/Volumes/Data prints, because that is the volume your files live on.

Every size quoted is allocated size, as reported by du. None of them is apparent size, as reported by ls. For an ordinary file the two agree; for a sparse file they can differ by hundreds of gigabytes, so only the allocated figure describes space the disk has actually given up.

Common questions

What is System Data on a Mac?

System Data is not a category of file. It is the bucket macOS uses for everything it cannot file anywhere else — caches, app containers, virtual machine disks, downloaded AI models, old device backups and local snapshots. Nothing prunes it automatically and macOS provides no way to look inside it.

How much space can clearing System Data free?

It depends entirely on what is inside it, so measure before assuming. On the one 512 GB MacBook Pro these guides are written from, working through it took the machine from 20 GB free to 215 GB free without deleting a single irreplaceable file. The 220 GB labelled System Data there turned out to be eight identifiable things, the largest being 92 GB of messaging media and 46 GB of dependency folders.

What is the best free alternative to CleanMyMac?

There is no single replacement. GrandPerspective and OmniDiskSweeper show what fills a disk, AppCleaner and Pearcleaner remove app leftovers, OnyX handles system maintenance, and Dustloft explains and reclaims System Data. All are free. There is a direct comparison with CleanMyMac, and a wider round-up of the free tools.