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What “System Data” really is on your Mac — and how to shrink it

Open Apple menu → System Settings → General → Storage and you will see a grey bar labelled System Data. On a lot of Macs it is the single largest slice — 50 GB, 100 GB, sometimes more — and macOS gives you no button to open it. It is not a bug and it is not a virus. It is a catch-all bucket, and once you know what is inside it you can reclaim most of it safely.

Updated 2026-08-02 · Written by the CleanerMac team · macOS 13–15 tested

What System Data actually contains

macOS sorts every file on your startup volume into a category: Applications, Documents, Photos, Music, Mail, iCloud Drive, macOS, Trash. Anything that does not fit one of those named buckets gets swept into System Data (older versions of macOS called the same thing Other). It is a leftover category, not a folder you can open in Finder.

In practice, the bulk of it is made up of these things:

• Caches — app caches in ~/Library/Caches, system caches in /Library/Caches, browser caches, and the media cache that Music, TV and Photos keep for streamed content.
• APFS local snapshots — Time Machine keeps hourly snapshots on your own disk when your backup drive is not connected. These are frequently the biggest single contributor.
• Logs and diagnostic reports — ~/Library/Logs and /private/var/log, plus crash reports that never get cleaned up.
• Mail downloads — attachments Mail has downloaded live under ~/Library/Containers/com.apple.mail and only sometimes count toward the Mail category.
• iOS and iPadOS device backups — stored at ~/Library/Application Support/MobileSync/Backup. A single iPhone backup can be tens of gigabytes, and old backups from devices you no longer own stay forever.
• Application Support and container data — Slack, Discord, Docker, Xcode, Adobe apps and virtual machines all park large working sets here.
• Swap and sleep image — /private/var/vm holds virtual-memory swap files that grow under memory pressure.
• Fonts, plug-ins, dictionaries and system extensions — small individually, meaningful in aggregate.

Note what is not in there: macOS itself. The operating system has its own sealed read-only volume and is reported separately as “macOS”. So a large System Data figure is never “just the OS being big”.

How to see the real number before you delete anything

The Storage screen is deliberately vague, so start with numbers you can trust. Open Terminal and run:

• df -h / — shows the true free space on your startup volume, including how much APFS considers available.
• du -sh ~/Library/* — lists the size of every folder in your user Library, sorted by nothing, so pipe it: du -sh ~/Library/* | sort -h. This is usually where the surprise lives.
• du -sh ~/Library/Application\ Support/MobileSync/Backup/* — shows each iOS backup individually.
• tmutil listlocalsnapshots / — lists every Time Machine local snapshot currently held on your disk.

du on a large Library can take a minute or two and will print permission errors for a few protected paths; that is normal and harmless. If you would rather not sit in Terminal, a visual breakdown does the same job — see our guide to using a Mac disk space analyzer.

Clearing the biggest offenders, in order

Work down this list. The first two items alone resolve most oversized System Data cases.

1. Purge Time Machine local snapshots. Check what exists with tmutil listlocalsnapshots /, then thin them aggressively with sudo tmutil thinlocalsnapshots / 999999999999 4. The huge number is a byte target for how much to reclaim and the 4 is the urgency level — the highest, which tells macOS to delete rather than negotiate. Your real Time Machine backups on the external drive are untouched.
2. Delete stale iOS backups. In Finder, connect the device and use Manage Backups, or delete folders directly from ~/Library/Application Support/MobileSync/Backup. Each folder is one device backup; the folder names are device identifiers, so check the modification date to work out which is which before removing anything.
3. Clear app caches. Quit the app first, then empty the contents of its folder in ~/Library/Caches. Delete the contents, not the folder itself, and never clear the cache of a running app. Full walkthrough in our guide to clearing the cache on a Mac.
4. Cut Mail's attachment downloads. In Mail, open Settings → Accounts → Account Information and set Download Attachments to Recent or None for IMAP accounts, since the server keeps the originals.
5. Remove old logs. Contents of ~/Library/Logs are safe to clear. Leave /private/var/log alone unless you know what you are doing — macOS rotates it on its own.
6. Uninstall properly. Dragging an app to the Trash leaves its Application Support, container and preference data behind, and all of that counts as System Data. See how to completely uninstall apps on Mac.

Where the space usually hides, by user type

Developers should pay particular attention to ~/Library/Developer/Xcode/DerivedData, which regenerates on the next build and is one of the safest large deletions on the whole machine — details in our Xcode derived data guide.

Why the number does not drop immediately

You delete 30 GB and the Storage bar barely moves. Three things explain that:

• The Trash still holds it. Nothing is reclaimed until you empty it.
• A snapshot still references it. If a local APFS snapshot was taken before the deletion, the blocks stay allocated until that snapshot expires or is thinned. This is also why deleted space often appears as purgeable space.
• The Storage pane caches its own numbers. Close System Settings entirely and reopen it, or check with df -h /, which never lies.

Keeping System Data small

The category grows back — that is its nature. A few habits keep it in check: connect your Time Machine drive regularly so snapshots get flushed to it rather than accumulating locally; delete an iOS backup as soon as you have restored from it; turn on Optimise Mac Storage for Photos and Messages if you use iCloud; and clear caches every few months rather than every few years. If your disk is already at the warning stage, start with our broader guide on how to free up space on Mac.

If you would rather not run Terminal commands or hunt through Library folders, this is precisely the job CleanerMac was built for: it shows the same breakdown visually, tells you in plain language which items are safe to remove, skips the caches of anything currently running, and sends files it removes from Disk X-Ray, duplicates and uninstalls to the Trash so those are reversible. It is a one-time purchase and runs entirely on your Mac.

What you should never delete

A short list, because a fast cleanup can turn into a bad afternoon: do not touch /System (it is sealed and read-only anyway), /private/var/db, /private/var/vm, the contents of ~/Library/Keychains, or any .plist in ~/Library/Preferences unless you are deliberately resetting a specific app. And never delete a Library folder wholesale — remove the contents of a specific subfolder instead, so the app can recreate what it needs.

See what is really inside your System Data

CleanerMac breaks it down file by file — and clears it safely.

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