What is System Data on a Mac?

Where to see it

Open System Settings, choose General, then Storage. The coloured bar at the top is Apple's picture of your disk. Hover over a segment and its name appears; the grey one, usually towards the right, is System Data. Below the bar Apple lists categories it can explain, with a button beside each. System Data has no button.

That is the whole problem in one screen. Apple names the block, gives it a size, and offers no way to look inside it. Deleting photos, emptying the Trash or restarting does not make it smaller, because none of those things are in it.

What is inside System Data

Apple does not publish the rule for what lands in the grey block, and there is no public interface that reports it. What can be said is what a scan of the places macOS and its apps write to actually finds. These are the categories MacMemory looks for, each in the one sentence the app itself uses. The first group is safe to delete: macOS or the app that made it will rebuild it, or it is a pure copy. The second is you decide: real data that is probably unwanted, but only you can say.

Safe to delete

• App caches — Temporary files apps keep to open faster. They rebuild themselves.
• Logs and crash reports — Text files apps write for their developers. You will never open them.
• Time Machine snapshots — Local copies macOS keeps between backups. It makes new ones anyway. More
• Mail downloads — Attachments Mail saved a second copy of. The originals stay in your mailbox. More
• Trash — Files you already deleted.

You decide

• Old iPhone and iPad backups — Full copies of devices, often ones you no longer own. More
• Old installers — Installers for apps you already have, old macOS installers, and disk images older than 30 days.
• Leftovers of deleted apps — Settings and support files apps left behind when you trashed them.
• Data from deleted apps — What sandboxed apps saved, still here after the app has gone.
• Developer leftovers — Xcode, simulators and package caches. Only shown if present. More
• Games you stopped playing — Games not played in two months. Steam downloads them again when you want one.

And the part nobody can remove

macOS needs this — The part of System Data that is macOS itself. Not selectable. Whatever is left of the grey block after everything above has been accounted for is the operating system’s own working space. It is not a fault and it is not yours to free up.

Six more things MacMemory can find are deliberately not in this list, because the app does not count them as System Data: “Docker images nothing uses”, “Packages in old projects”, “Simulators you stopped using”, “Large files”, “Duplicates”, “Apps you installed”. They live in your own folders, in Applications and inside Docker’s own storage.

Why System Data grows

Most of the grey block is made of things that are supposed to exist. Apps keep caches so they open faster and rebuild them when they are gone. Time Machine keeps snapshots between backups so it can restore a file you changed an hour ago, and it makes new ones on its own. Mail saves a second copy of an attachment when you open it. A phone you backed up to this Mac in 2021 is still here in full, whether or not you still own the phone.

None of that is a fault. It becomes a problem only when the disk is nearly full, because macOS does not clear most of it until it is under pressure, and by then the warning has already appeared. The block is not one thing that got big. It is a dozen ordinary things that were never shown to you.

This also explains why the usual advice does not work. Deleting photos shrinks the Photos segment. Emptying the Trash shrinks the Trash. Neither touches a cache, a snapshot or an old backup, so the grey block stays the size it was.

What not to touch

Some of what a search for “delete System Data” turns up is dangerous. The rule that keeps you safe is simple: if you cannot say in one sentence what a folder is for and what will rebuild it, leave it alone.

• Anything under /System or /Library. The first is the sealed operating system; the second is shared support files for every user of this Mac. Neither is yours to trim.
• Your Photos library and your Mail mailboxes. The folder beside Mail’s downloads holds the mailbox itself. Emptying it deletes mail.
• Other users’ folders. A Mac with two accounts has two homes.
• Snapshots that are not Time Machine’s. macOS keeps its own snapshots around system updates. They look similar in a list and removing one is system surgery, not freeing up space. The snapshots page shows how to tell them apart.

This is also why MacMemory has two labels and no third. Anything the app judges risky is not shown at all, so there is nothing on its screen a red warning would be needed for.

Common questions

Is System Data the same as "Other"? Yes. Older versions of macOS called the grey block Other. It was renamed System Data in macOS Monterey and the contents did not change.

Can System Data be reduced to zero? No. Part of it is the operating system's own working space, which is why the app's list ends with a row called "macOS needs this" that cannot be selected.

Will it come back? Some of it, yes, and that is fine. Caches rebuild as you use apps; Time Machine makes new snapshots on its own. What does not come back is what you chose to remove: an old backup, an installer, the settings of an app you trashed years ago.

Does restarting help? A restart clears some temporary files and gives macOS a chance to remove snapshots it no longer needs. It does not touch backups, installers, leftovers or Mail's copies, which is where most of the space usually is.

How to free up System Data without an app

Everything below can be done with Finder, System Settings and Terminal, on macOS 14 and later. Each step says what it removes, whether it is safe to delete or yours to decide, and what undo looks like.

1. Empty the Trash. Safe to delete

   Files you already deleted still take space until the Trash is emptied. Control-click the Trash in the Dock and choose Empty Trash.

   Undo: None. Emptying the Trash removes the files outright; look through it first.

2. Review old iPhone and iPad backups. You decide

   Connect the device, open it in Finder's sidebar, choose Manage Backups, select a backup and choose Delete Backup. Without the device, the backups are folders you can open in Finder and move to the Trash. If you still own the device, back it up to iCloud first.

   Location: ~/Library/Application Support/MobileSync/Backup

   Undo: A backup moved to the Trash can be put back until the Trash is emptied. Once emptied, that copy of the device is gone.

   More about old backups

3. Remove Time Machine snapshots. Safe to delete

   In Terminal, run tmutil listlocalsnapshotdates / to see the Time Machine snapshots on your startup disk, then delete one by its date with the command below. Only dates returned by the first command are Time Machine's; anything else in a snapshot list belongs to macOS and stays. No password is asked for, and Time Machine makes new snapshots on its own. Removing these does not affect backups on your backup disk.

   tmutil deletelocalsnapshots 2026-09-01-120000

   Undo: None. A snapshot is removed immediately. The backups on your backup disk are unaffected.

   More about time machine snapshots

4. Clear Mail's attachment copies. Safe to delete

   Mail keeps a second copy of every attachment you open. The originals stay in your mailbox. Open the folder below in Finder and move its contents to the Trash. Do not touch the Mail folder in Library itself, which is the mailbox.

   Location: ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads

   Undo: In the Trash until you empty it. Opening the attachment in Mail again makes a fresh copy.

   More about mail downloads

5. Clear app caches. Safe to delete

   Apps keep temporary files so they open faster and rebuild them when they are gone. Open the Caches folder in Finder, select its contents and move them to the Trash, then quit and reopen the apps you use. Some apps will sign you out or take longer to open once.

   Location: ~/Library/Caches

   Undo: In the Trash until you empty it. Apps rebuild what they need on the next launch.

6. Clear logs and crash reports. Safe to delete

   Text files apps write for their developers. You will never open them. Move the contents of the Logs folder to the Trash.

   Location: ~/Library/Logs

   Undo: In the Trash until you empty it. Apps write new logs as they run.

7. Remove old installers. You decide

   Disk images and installers in Downloads older than 30 days have usually done their job. Sort Downloads by kind, look for disk images and installer packages, and move the ones you have already installed to the Trash.

   Location: ~/Downloads

   Undo: In the Trash until you empty it. An installer can be downloaded again from wherever it came from.

8. Remove developer leftovers, if you have Xcode. You decide

   Only if a developer has used this Mac. Xcode's build files rebuild on the next build, and simulator runtimes you no longer target can be removed from Xcode's settings. The developer page lists each location.

   xcrun simctl delete unavailable

   Undo: Xcode rebuilds its build files on the next build. A removed simulator runtime is downloaded again from Xcode's settings.

   More about developer leftovers

9. Let macOS suggest the rest. Safe to delete

   Back in System Settings, General, Storage, Apple lists its own recommendations above the categories: store files in iCloud, empty the Trash automatically, review large files. They are conservative and they are safe to follow.

   Undo: Each recommendation is a setting you can turn off again.

Read next

• Time Machine snapshots and System Data
• Old iPhone and iPad backups on your Mac
• Developer leftovers: Xcode, simulators and package caches
• Mail downloads and System Data