What's Taking Up Space on My Mac?
Your Mac's storage is full, but you don't know why. Here's a breakdown of the real culprits — including what 'System Data' actually means.
The Problem With macOS's Built-In Storage View
Open Apple menu → System Settings → General → Storage (on macOS 12 Monterey and earlier, Apple menu → About This Mac → Storage) and you'll see a bar divided into broad categories: Applications, Documents, Photos, System Data. The problem is these categories are too vague to act on — and "System Data" in particular can balloon to 40 GB with no explanation and no way to drill down.
To actually understand your storage, you need a tool like LumaDisk that shows you every file and folder with its real size — not approximations grouped into buckets.
The Most Common Culprits
Through disk analysis, the same categories of files show up again and again as the biggest storage consumers:
Large media files
Photos, videos, screen recordings, and podcasts are usually the biggest consumers. Raw video files in particular can reach tens or hundreds of gigabytes. Check your ~/Movies/ and ~/Downloads/ folders first.
Old backups and disk images
iPhone and iPad backups stored in ~/Library/Application Support/MobileSync/ can reach many gigabytes per device, and most people have multiple old backups sitting around. Disk images (.dmg files) from software installers also accumulate over time.
Application caches and support data
Browser caches, video streaming download caches, and application support data live in ~/Library/Caches/ and ~/Library/Application Support/. These grow silently — it's common for a browser cache alone to reach several gigabytes.
Build and tool caches
Even if you're not a developer, tools like Docker, npm, or CocoaPods may have created caches on your machine if you've ever installed them. Xcode's DerivedData directory is a notorious space hog — it can quietly grow to 20–30 GB over time. LumaDisk flags these immediately: its build-cache detection groups exactly this kind of clutter — node_modules, /target, .next, .gradle, and DerivedData — so you can reclaim gigabytes in one pass.
Local Time Machine snapshots
When your backup drive isn't connected, macOS stores local Time Machine snapshots to sync later. These are automatically managed and deleted when space is needed, but they can inflate reported storage usage. You can delete them manually with tmutil deletelocalsnapshots / in Terminal if needed.
What Is "System Data"?
System Data in macOS's storage view is a catch-all category for files the OS doesn't put elsewhere. It typically includes:
- Application caches and support files
- Log files generated by apps and the system
- Local Time Machine snapshots
- Virtual machine disk images
- Browser data and extensions
- Temporary system files
macOS doesn't give you any way to see what's inside System Data through its standard tools. A third-party disk analyzer is the only way to see a real breakdown — which is exactly what LumaDisk is built for.
How to See the Real Breakdown
The fastest way to understand your Mac's storage is with a disk space visualizer. LumaDisk scans your disk and renders every file and folder as a wedge in an interactive sunburst chart, sized to the space it occupies. Folders taking more space appear as bigger wedges; click any wedge to drill into it, with a breadcrumb trail keeping you oriented. Within seconds you can see exactly which folder is the culprit — no more guessing.
Unlike macOS's built-in storage view, LumaDisk shows actual file sizes rather than approximations, and there are no catch-all categories hiding the truth.