Mac System Data storage explained: what it is and what is safe to delete

System Data is the catch-all category in Mac Storage settings. Apple describes it as the space used by all the Apple and third party files that do not belong to a more specific category. In practice that means caches, logs, app support data, local Time Machine snapshots, device backups, developer files and more. Apple does not give you one button to clear it. You can shrink it by finding what is big and removing the things that get rebuilt, and the rule for all of it is to move things to the Trash first so you can put them back.

What is in it

The mix differs from one Mac to the next. Common contents:

• Caches and logs from macOS and your apps, in the Library folders.
• Application support data, including large app libraries.
• Local Time Machine snapshots on the startup disk.
• iPhone and iPad backups, if you back up to the Mac.
• Developer files such as Xcode build data, device support files and simulators.
• Virtual machine images, fonts and plug-ins.

Depending on your macOS version, some of these, such as developer files or backups, may be listed under their own line in Storage instead. The size also moves around on its own after updates and restarts.

Apple says Time Machine takes a local snapshot about every hour, keeps it for 24 hours, and removes snapshots automatically as they age or when the system needs the space. Apple also says the space they use counts as available storage, so macOS can free it when it needs the room. Apple's way to remove them early is to turn off automatic backups for a few minutes. See About Time Machine local snapshots.

See what is big without deleting anything

Start in System Settings, General, Storage. Apple lists this and other ways to free space on its page Free up storage space on Mac.

For more detail, these Terminal commands only read:

# the biggest folders inside your Library
du -h -d 1 ~/Library | sort -hr | head

# the two usual suspects
du -sh ~/Library/Caches ~/Library/Logs

# app data, largest first
du -h -d 1 ~/Library/Application\ Support | sort -hr | head -15

# iPhone and iPad backups
du -sh ~/Library/Application\ Support/MobileSync/Backup

# local Time Machine snapshots
tmutil listlocalsnapshots /

Terminal may report "Operation not permitted" for some folders unless it has Full Disk Access. The sizes can differ a little from Finder's. A visual tool is often easier. DaisyDisk (paid), GrandPerspective, OmniDiskSweeper and the command line tool ncdu all show what is taking space.

What is safe to delete

These are usually fine to remove, one at a time, with the app quit:

• A single app's folder in ~/Library/Caches. The app rebuilds it, and its first launch may be slower.
• Old logs in ~/Library/Logs.
• Xcode's DerivedData folder, if you build apps. Xcode regenerates it.
• Installers you already used (.dmg and .pkg files) in Downloads.
• node_modules and build folders in projects you no longer work on. A package install rebuilds them.
• Old iPhone and iPad backups, but only after checking the date and device, because a backup can be the only copy of something.

What to leave alone

• Anything under /System, /Library or /private, including the swap files in /private/var/vm.
• Application Support, Containers and Group Containers for apps you still use. They can hold documents, settings and history.
• Mail, Messages and Photos data.
• iCloud Drive and other synced folders. Deleting there deletes the cloud copy too.
• Anything you cannot identify.

Move to the Trash, do not rm

Use Finder and move items to the Trash. Use the Mac for a few days, then empty it. If something breaks, use Put Back. Avoid sudo rm -rf for this job. It skips the Trash, and one wrong path cannot be undone.