How to Clear System Data on Mac: It's Not Your Caches

Your Mac says the drive is full. You open Storage, and a grey bar labelled System Data is sitting on 80GB, 150GB, sometimes more. Almost every guide on how to clear system data on Mac tells you to empty your Caches folder, and most of the time that gets you almost nothing back.

I have been through this on two Macs now and the answer was the same both times. The space was not in caches. It was in APFS local snapshots, which macOS creates quietly, holds on to for far longer than you would expect, and never shows you anywhere in System Settings.

Here is what System Data actually contains, the two commands that shift most of it, and the advice on page one that you should ignore.

What System Data on a Mac actually is

Apple's own definition is deliberately vague. Its support page on freeing up storage space describes System Data as covering files that "don't belong to any more specific category". So it is not a thing. It is a leftover.

Anything macOS cannot confidently label as Applications, Documents, Photos or Music lands in that bucket. That means System Data has no single cause and no single fix. Two Macs both showing 120GB can be holding completely different things.

Chasing the number down without finding out what is in it is how people end up deleting files they needed. So the first job is not deleting. It is measuring.

Why the cache advice on page one is mostly wrong

Search this question and the top results are a Reddit thread about macOS Monterey, an Apple Discussions thread from January 2023, and a personal blog post. All three tell you the same thing: open Finder, press Command-Shift-G, go to ~/Library/Caches and delete everything inside.

There are two problems with that. The first is that user caches are usually small. On my machine the whole folder is a few gigabytes, and most of it is browser and Xcode data that regenerates within a day. You get the space back for about an hour.

The second is that it can break things. Emptying caches while apps are running leaves some of them confused, and a handful of apps store data in Caches that they really should not: unsynced drafts, downloaded assets, licence files. It is a low-reward move that is not risk-free.

How to clear system data on Mac: the snapshots nobody mentions

When Time Machine cannot reach your backup drive, because you have unplugged it or because you never set one up, macOS takes a snapshot of the volume and keeps it on the internal drive instead. These are meant to be temporary. In practice they can sit there for weeks, holding on to every file you have deleted since they were taken.

What makes them so hard to spot is that they appear nowhere in System Settings. Apple's own guide to freeing up storage does not mention local snapshots once - I checked it again while writing this. Disk Utility will show your drive as fuller than the sum of its visible files, and nothing on screen explains why.

How to see how much space snapshots are holding

Open Terminal and run this:

tmutil listlocalsnapshots /

You get back a list of snapshot names, each one stamped with the date and time it was taken. If you see more than a couple, or dates going back weeks, that is where your System Data has gone. Open Disk Utility alongside it and look at the Purgeable figure in the capacity bar - that number is mostly these.

How to delete local snapshots on a Mac

To remove a single snapshot, take its date stamp from the list above and run:

sudo tmutil deletelocalsnapshots 2026-08-20-104500

To clear them in bulk and let macOS reclaim as much as it can in one go, run this instead:

sudo tmutil thinlocalsnapshots / 999999999999 4

That second command asks macOS to free roughly a terabyte of snapshot data from the startup volume at urgency level 4, which is the most aggressive setting available. It frees what it can and stops. Howard Oakley's reference on tmutil is the clearest write-up of the syntax if you want the full set of options.

Neither command touches your live files. A snapshot is a record of a past state, not the only copy of anything currently on your drive. You will be asked for your password, and depending on your macOS version the delete may refuse without sudo in front of it.

What a normal System Data figure looks like

Nobody on page one will give you a number, so here is mine. On a clean install of macOS 26 Tahoe with a normal set of apps and no developer tools, System Data settles somewhere between 15GB and 40GB. Sixty gigabytes is worth investigating. Above 100GB means something specific has gone wrong, not that macOS is bloated.

Aiming for zero is the wrong goal. A working Mac needs swap files, a sleep image roughly the size of your RAM, font caches and system logs. Some of what is sitting in that bar is doing a job, and clawing at it will only make your Mac slower.

The other five things that inflate System Data

If the snapshots were not the problem, or you cleared them and the bar is still fat, these are the usual suspects in the order I check them.

• Xcode. Derived data, device support files and simulator runtimes. Running xcrun simctl delete unavailable removes orphaned simulators and regularly returns tens of gigabytes on a machine that has had Xcode installed for a few years.
• Old device backups. iPhone and iPad backups made to the Mac rather than to iCloud live in ~/Library/Application Support/MobileSync/Backup and are never cleaned up automatically.
• Leftover macOS installers. A full installer is 15GB or so and sits in Applications after an update until you drag it to the Bin yourself.
• Disk images. Every .dmg you mounted once and forgot about, usually in Downloads.
• Mail attachments. If Mail is set to download all attachments for offline use, a decade of them adds up quietly.

What macOS Tahoe changed, and what it did not

Through the macOS 26 Tahoe beta cycle there was a run of reports about free space disappearing entirely - people watching hundreds of gigabytes vanish with nothing in Storage to account for it. Michael Tsai collected the reporting and the suspects were a runaway Spotlight indexing process and, again, accumulated snapshots.

I want to be straight about the status of that: it is forum reporting and developer anecdote, not something Apple has confirmed or documented. Treat it as a reason to check your snapshot list rather than a diagnosis. If your Mac started eating space right after a macOS update, the two commands above are still the first thing to try.

What Tahoe did not change is the Storage screen itself. It is still System Settings, General, then Storage in the sidebar on the right, and it still gives you a coloured bar with no way to interrogate the grey part of it. If you like poking at the parts of macOS Apple does not advertise, I have written about the hidden Mac apps Apple never mentions and about getting a Mac to power itself on in the same spirit.

What that reclaimed space is actually worth in pounds

Say you get 60GB back, which is roughly what the snapshot clear-out returned on my Mac. What would it cost to buy that headroom instead of reclaiming it?

The nearest thing to buying storage from Apple without replacing the machine is iCloud+. Apple UK charges £0.99 a month for 50GB and £2.99 a month for 200GB, so shifting 60GB of files off the drive means the 200GB tier. That is £35.88 a year, every year, for space you already own (my arithmetic, at today's apple.com/uk prices).

The other way to look at it is proportional. On a 256GB MacBook Air, 60GB is 23 per cent of the entire drive (again, my arithmetic). That is the real cost of leaving snapshots in place. Not an abstract number in a grey bar, but very nearly a quarter of the machine you paid for.

What to leave alone

• The /System folder and /Library/Caches at root level. You will be told to empty the second one. Do not, unless you know exactly which app put what in there.
• Anything inside ~/Library you cannot identify. If you cannot name the app that created it, you cannot know what breaks.
• The sleep image and swap files. macOS rebuilds them immediately and you gain nothing.
• "Cleaner" apps that promise to clear System Data. Most of them empty the same caches page one told you to empty, then charge you for it. If you want Mac apps that actually earn their place, I keep a running list.

Frequently asked questions

Why does System Data on my Mac keep growing back?

Because most of what you deleted was cache, and macOS rebuilds cache. If the figure climbs back within a day or two, you cleared the wrong thing. Snapshots do not regenerate the same way, so a genuine snapshot clear-out holds unless Time Machine loses sight of your backup drive again.

Is it safe to delete System Data on a Mac?

Deleting local snapshots is safe, because a snapshot is a record of a previous state rather than your only copy of anything. Deleting the contents of system cache folders by hand is not reliably safe, which is why safe mode exists - starting up in safe mode clears certain system caches for you, in the order macOS expects.

Does restarting a Mac clear System Data?

A normal restart clears very little. Booting into safe mode clears some system caches and is the safer version of the manual cache deletion everyone recommends. Neither one touches snapshots, which is why the figure so often survives a reboot.

My take

The reason this question has such a bad SERP is that the honest answer is boring. There is no single button, and the biggest cause is invisible from the interface Apple gives you. So people write up the one thing they can see, which is the Caches folder, and everybody copies it.

Run tmutil listlocalsnapshots before you delete anything. Nine times out of ten it tells you the whole story in about four seconds, and you can leave your caches exactly where they are.

Source: Free up storage space on Mac, Apple Support