See what's inside your Mac's System Data

What macOS counts as System Data

In System Settings › General › Storage, System Data is the grey part of the bar that has no breakdown. It's where macOS puts everything that doesn't fit its other categories. On a typical Mac that includes:

• App caches and logs, and crash and diagnostic reports.
• Time Machine local snapshots: hourly snapshots kept for 24 hours, plus one of your last backup until the space is needed.
• Downloaded macOS updates and other macOS assets, plus the swap file and sleep image.
• Virtual machines and container disk images, such as Docker Desktop, OrbStack and UTM.
• iPhone and iPad backups, and Mail and Messages attachments, depending on your macOS version.
• Developer data: Xcode DerivedData, simulators and package caches.
• Data apps keep in their containers.

Because it mixes things that are safe to remove with things you must keep, deleting "System Data" isn't a single action.

How DiskFox breaks it down

DiskFox measures the locations macOS counts as System Data and adds them up into an estimate. It shows that estimate on the Home screen and as a dashed line in Storage Trends. For each part, it tells you what it is and what it can do about it:

Part of System Data	What DiskFox does
App caches, logs, crash reports, old temporary files	Measures them; can move them to the Trash after you review
Developer data (Xcode, simulators, package caches)	Measures them; most can move to the Trash, and simulators use Xcode's own tools
Docker Desktop, OrbStack, UTM and other VMs	Measures them; gives a guide, since these are best trimmed with their own tools
iPhone and iPad backups, Mail attachment copies	Measures them; old backups can move to the Trash after review
Downloaded macOS updates, swap and sleep image	Measures them; protected, because macOS manages them
Time Machine local snapshots	Counts them (macOS doesn't report each one's size); gives a guide
Purgeable space	Shows it; macOS frees it automatically when needed
Space it can't place	Shown as "space DiskFox hasn't attributed", never guessed

Some of these, such as backups, Mail, Messages and VMs, can only be measured with Full Disk Access, which is optional.

Why System Data grows

System Data usually jumps for a reason you can find: a macOS update downloading, a virtual machine growing, Time Machine local snapshots, or developer tools building up caches. DiskFox's What Changed view shows which part grew and when, and labels the likely cause. The full troubleshooting steps, including what you can check without DiskFox, are in our guide: Why is my Mac's System Data so large?

What's safe to remove

• Usually safe: caches, logs and old temporary files. Apps rebuild what they need, so some of it comes back over time.
• Safe after a look: old iPhone backups, Xcode archives and unused simulators. Keep anything you might still need.
• Leave to macOS: macOS updates, the swap file and sleep image, purgeable space and Apple Intelligence models. See how much space Apple Intelligence takes.

DiskFox only moves items to the Trash after you review them (how its cleanup works), and never removes anything on its own.

How accurate is it?

DiskFox's System Data figure is an estimate built from what it can measure. It can differ from the number in Storage settings, because macOS doesn't publish exactly what it counts, and some locations need Full Disk Access. DiskFox shows the part it can't attribute rather than hiding it.