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. If you already know you are hunting for individual big files rather than a category, our guide on how to find large files on Mac covers Finder, Terminal and the faster options.
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, and 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, free to download with a 7-day trial of everything, 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, with 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. Scanning stays free after the trial, and the full app is a one-time $6.89.
Frequently asked questions
Why is System Data so large on my Mac?
System Data is a catch-all for files macOS does not sort into another category: app caches, logs, local Time Machine snapshots, virtual machine images and temporary files. It commonly reaches 20 to 40 GB, and it grows when snapshots pile up while your backup drive is disconnected or when a cache is never cleared.
Is it safe to delete System Data?
You cannot delete the category itself, only what is inside it. User caches in ~/Library/Caches rebuild themselves and are generally safe to clear, and local Time Machine snapshots can be removed with tmutil. Never delete anything inside /System or /Library/Apple, and quit an app before clearing its cache.
How do I see what is inside System Data?
macOS Storage settings will not open the category. A disk space analyzer scans the actual folders instead, so the caches, snapshots and VM images behind the number appear as individual folders with real sizes.
Why is my Mac storage still full after deleting files?
Usually one of three things: the Trash has not been emptied, local Time Machine snapshots still hold the deleted data, or macOS is counting purgeable space it has not reclaimed yet. Emptying the Trash and restarting releases most of it.