system data: 180 gb, contents unspecified

where macOS shows system data, and what it will not tell you

1. System Settings → General → Storage. the screen that shows you the number, and its Recommendations are worth doing first — emptying the trash automatically, removing watched Apple TV downloads.
2. then, in Finder, ⇧⌘G and go to ~/Library. your share of it lives there, hidden precisely because most of it is not meant to be edited by hand.

Storage will not itemise System Data for you, and that is the whole reason this question exists. Apple's own description of the category is that it «measures the storage space used by all Apple and third-party files that don't belong to any more specific category» — which is an honest way of saying «the remainder».

Apple's definition of the category, in its own words: free up storage space on Mac — Apple Support

why df and the storage screen disagree

before deleting anything, know that two of your numbers are meant to differ. macOS reports free space twice: what is physically unused, and a larger figure that adds purgeable space — files it will throw away by itself the moment you need the room. on the machine this page was written on the gap was six gigabytes, 45.7 GB against 51.8 GB. df -h /System/Volumes/Data gives you the first, the Storage screen leans towards the second, and those six gigabytes were never yours to reclaim because they were never taken.

ignore the advice, repeated across dozens of articles, that diskutil info / prints a «Purgeable Space» line. run it and grep for the word: there is Container Free Space and nothing else. purgeable is a value the filesystem hands to applications through a resource key, not a field in a disk report.

the key applications actually read for this: volumeAvailableCapacityForImportantUsageKey — Apple Developer

the same gap is what inflates the figure a cleaning app reports after a scan: what the space a cleaner finds is made of

what system data is actually made of

• Time Machine local snapshots. usually the largest single item — when they exist at all, which is the part nobody mentions.
• iOS and iPadOS backups, in ~/Library/Application Support/MobileSync/Backup. tens of gigabytes each, one per device, kept forever. delete them from Finder's device screen, not by hand.
• caches, in ~/Library/Caches. every app you use, and every app you removed years ago without its leftovers. safe to clear, rebuilt on demand, and the next launch of everything is slower.
• installers nobody ever cleaned up — .dmg and .pkg files in Downloads, on the Desktop and in Documents. dull, and frequently double-digit gigabytes.

snapshots: check the right volume first

start with tmutil destinationinfo. if it answers «No destinations configured», you have no snapshots at all and this branch is closed for you: Apple stores them only when a backup disk is set up and Back Up Automatically is on. five seconds, and it saves the afternoon people spend on the most-recommended fix on the internet.

if you do have them, the command everyone quotes — tmutil listlocalsnapshots / — prints a heading and nothing else, because snapshots live on the data volume: tmutil listlocalsnapshots /System/Volumes/Data. macOS takes one about every hour, keeps it twenty-four, and frees them under pressure, so the number alarms you long before it costs you anything. deleting one by hand with tmutil deletelocalsnapshots is rarely the right move.

the retention rules and the condition for having any: about Time Machine local snapshots — Apple Support

why du will not measure your ios backups

the standard way to size a folder is du -sh, and on MobileSync it fails: du: Operation not permitted. so does ls. the folder sits behind the privacy layer and Terminal does not hold that permission by default — nothing is broken, and no variation of the command will get through. read the size in Finder on the device screen instead, and delete old backups from there. granting Terminal full disk access would also work, and handing a shell permanent access to everything to measure one folder is a poor trade.

what to leave alone: anything in ~/Library/Application Support belonging to an app you still use — that is where licences, local databases and years of your own work live.

the leftovers of apps you did remove are a separate job, with its own rule about bundle identifiers: uninstall an app with its leftovers

and if the app itself refuses to go: delete an app that will not delete

the part of system data nothing can clean

your disk carries volumes besides the two you think about. run diskutil apfs list and you will find them counted, invisible in Finder, and filed under System Data. on the machine this page was written on:

• Preboot — 9.1 GB here, which is on the high side; a few gigabytes is more typical. it holds what the Mac needs before the system volume is unlocked.
• Recovery — 1.3 GB. the environment you boot into when the main system will not start.
• VM — 8.6 GB, and this one moves. it is swap: eight one-gigabyte files here, written as memory ran short. it grows from what you have open, not from what you have saved.

if it is the VM volume that keeps growing, the disk is not the problem: fix «your system has run out of application memory»

that is roughly nineteen gigabytes, and no cleaning app removes any of it, ours included. the system volume is sealed and verified at boot, Preboot and Recovery are what make that possible, and swap belongs to the kernel. an application deleting these would be a bug, not a feature.

so, before going further: run tmutil destinationinfo, subtract the volumes above, and treat six or so gigabytes of the rest as purgeable and already yours. what survives that subtraction is the part worth acting on — caches, old installers, device backups and the leavings of apps that are no longer here. often thirty or forty gigabytes, and it is filing, not detective work.

why the system volume is not yours to prune: what is the sealed system volume — Apple Support

how hop clears the system data it can reach

that filing job is four Finder searches, a Terminal command per folder you want measured, and knowing which folders those are. hop's uninstall module makes the pass without removing any apps, largest thing first. what it does not do is the other half of this page, and that belongs in the table rather than in a footnote you find later.

the module, and what it looks for: app uninstaller for Mac

• one tick takes a whole section, and nothing is deleted: it all moves to the Trash, so a mistake costs a restore rather than a file.
• what it leaves alone is listed too — a container where cache and data share one folder, a messenger's twenty gigabytes among them, because only that app's own cleanup knows which half is disposable. a cleaner that takes those without asking is how people lose message history.

when the system data number does not move

• you deleted things and the figure is unchanged. the Storage screen recalculates slowly — wait a few minutes, or check with df -h, which does not cache.
• it went down and came straight back up. snapshots, and they are supposed to. they are freed when the disk is genuinely under pressure.
• the Trash was never emptied. deleted files count until it is.
• an app is holding a deleted file open, so the space is not returned until it quits. a log out settles this without a reboot.
• it is Photos or Mail, filed here because of how their caches are stored. both clean up from inside the app, and hand-deleting their libraries loses things permanently.
• the disk is full enough to misbehave. below about 10% free, macOS starts making its own decisions and every measurement gets strange.

Xcode, simulators and the 200 gb question

if the number is two hundred gigabytes rather than forty and you write software, stop reading general advice — this is its own case, and it fills the developer forums after every macOS release. simulator runtimes are kept per version and survive upgrades, derived data grows without limit, device support accumulates a folder per iOS build, and archives are kept forever by design.

the runtimes come off in the same Storage screen this page opened with — System Settings → General → Storage → Developer — which most people have never scrolled to. xcrun simctl delete unavailable is a different job: it clears simulator devices that can no longer launch, not the runtimes underneath them. the rest lives under ~/Library/Developer, which du can measure because, unlike MobileSync, it is not behind the privacy layer. no general-purpose cleaner should be trusted with any of it.

Apple's own thread on where Xcode's space goes: FYI: Xcode and disk space — Apple Developer Forums

and if the machine is slow with it, the disk may be the symptom: what memory pressure actually measures

frequently asked questions

what is system data on a Mac?

the remainder category on the Storage screen: everything not counted as apps, documents, photos, music or mail. mostly caches, logs, device backups, local snapshots and the hidden volumes macOS needs to boot, shown without a breakdown.

is it safe to delete system data?

parts of it. caches, old installers and old iOS backups are safe. the sealed system volume, Preboot, Recovery and the application-support folders of apps you use are not.

why is system data so large after a Time Machine backup?

local snapshots. macOS keeps them on the internal disk between backups and counts them here. they are released automatically when space is needed, so a large number on a disk with room is not a problem.

how do i delete local snapshots?

list them with tmutil listlocalsnapshots /System/Volumes/Data — the data volume, not /, where most guides send you and where you will see nothing. tmutil deletelocalsnapshots with a date removes one, and it is rarely necessary.

why does Terminal say operation not permitted on my backup folder?

MobileSync sits behind the privacy layer and Terminal has no access to it by default. check the size in Finder on the device screen, which is also where you delete old backups safely.

does clearing caches speed up a Mac?

no. caches exist to make things faster, and clearing them makes the next launch of everything slower. clear them for space, not for speed.

how much free space should a Mac have?

aim for around ten per cent. below that, swap runs out of room and the «out of application memory» warnings start on machines with plenty of ram.

the tool behind this

done with uninstall apps

other things people do

• the app is gone. its eighteen folders are not.app uninstaller
• the app will not go in the trashapp uninstaller
• the app is gone. its system extension is still there.app uninstaller