What is "System Data" on a Mac, and how do you shrink it?

A Diskscape guide · November 2026

"System Data" is everything macOS can't put in another category in Settings › General › Storage. It's often tens of gigabytes, and most of it shrinks on its own. Here's what's in it, what you can safely do, and what to leave alone.

What's inside

Apple describes System Data as files the system uses: logs, caches, virtual memory, temporary files, fonts, app support files and plug-ins. In practice, the big pieces are usually these:

• Swap and the sleep image. When memory runs short, macOS moves some of it to disk. This shrinks after a restart.
• Local Time Machine snapshots. If you use Time Machine, your Mac keeps recent snapshots on the internal disk so you can restore files even without the backup drive. macOS removes them by itself when space runs low.
• Caches and logs. Apps and the system keep these to work faster or to record problems. Most rebuild on their own.
• Startup and system volumes. Files your Mac needs to start up, plus downloaded macOS updates waiting to install.
• Anything a storage scan can't attribute. Folders macOS doesn't let apps read, and app data that doesn't fit another category.

Step 1: restart

It sounds too simple, but a restart clears swap, temporary files and some caches. If System Data dropped a lot afterwards, most of it was memory spilling onto the disk, and the long-term fix is closing memory-hungry apps or tabs.

Step 2: check local snapshots

Open Terminal and list the snapshots on your startup disk:

tmutil listlocalsnapshots /

Each line contains a date such as 2026-11-02-091530. To delete one snapshot, use that date (macOS may ask for your password):

sudo tmutil deletelocalsnapshots 2026-11-02-091530

This only removes the local copy. Backups on your Time Machine drive are not affected. Since macOS removes local snapshots by itself when space is needed, you rarely need to do this unless an app insists there isn't enough room.

Step 3: look for big folders that hide here

Some large items often get lumped into System Data or end up unaccounted for. These are worth checking, but they hold real data, so look before you delete:

• iPhone and iPad backups. In Finder, select your device and click Manage Backups. Keep the latest backup of any device you still use.
• Xcode and iOS Simulators. Build data rebuilds on its own; simulators hold apps you installed in them. See our guide to developer caches.
• Virtual machines and Docker. Parallels, UTM and Docker store whole systems in single large files. Deleting one erases everything inside it.

What not to do

• Don't delete folders inside /System, /private/var or /Library by hand. macOS relies on them, and some are protected anyway.
• Don't run "clean everything" scripts you found online. They often delete things that are expensive to get back.
• Don't panic if System Data grows for a while: snapshots and caches come and go.

How Diskscape shows it

Diskscape draws System Data as a grey block on the map, so you can see its real size next to everything else. Click it for a breakdown into swap and sleep image, startup and security files, update staging and the rest, plus a list of the folders macOS wouldn't let it read and whether Full Disk Access would reveal them.

Sources

• man tmutil on macOS (listlocalsnapshots, deletelocalsnapshots)
• Apple Community: How to delete the Time Machine's local snapshots
• Macworld: How to delete System Data on a Mac
• The Eclectic Light Company: Storage crisis management