What is System Data on a Mac, and how much of it can you get back?

Open System Settings › General › Storage on a Mac that has been in use for a few years and you may find a grey bar labelled System Data taking 60, 80 or 120 GB. Click it and macOS tells you very little. There is no list and no delete button.

That is because System Data is not a place. It is the name macOS gives to whatever is left after it has counted your apps, documents, photos and so on. To shrink it, you have to work out what is actually in it on your Mac.

What usually fills it

The common contents, roughly in the order we would suspect them:

• Caches and logs. Written by every app you use, in your Library folder.
• App support files. The data apps keep in ~/Library/Application Support, including apps you deleted long ago.
• Time Machine local snapshots. Kept on the internal disk between backups.
• Virtual disk images. Docker's Docker.raw and any virtual machines. One file, often tens of gigabytes.
• Developer leftovers. Xcode's DerivedData, simulators and device support files.
• Swap and other temporary files that macOS manages itself.

Step one: check the part you should leave alone

Open Terminal and run:

tmutil listlocalsnapshots /

A long list explains some of the grey bar. It is also the part that needs no action from you, because macOS removes these snapshots when the disk needs the room. If the bar looks alarming but you still have plenty of free space to work with, this may be all that is going on. SweepyMac does not offer to delete snapshots, and we think that is the right call for a cleaner.

Step two: take the caches and logs

These are the safe wins. Smart Scan in SweepyMac sweeps user caches, application logs and saved app state in one pass and totals what it can reclaim. The full Cleanup module adds developer caches and the support files of apps you have removed, listed but left unticked for you to decide. Our post on Smart Scan and Cleanup explains the difference.

Everything goes to the Trash, and files in the Trash still count. Your System Data will not move until you empty it, which is worth doing a day or two later, once you are sure nothing is missed.

Step three: look inside your Library

This is where a map beats a list. Point Space Lens at your Library folder and drill down: Application Support is usually the largest bubble, and one app's folder inside it often turns out to be the whole story. You will spot an old video editor's render cache, a chat app's media store, or a game launcher you removed in 2023.

Space Lens will not let you remove Library as a whole, however tempting the bubble. Drill in and take the specific item. If the item belongs to an app you still have, remove it through that app's own settings if you can. If you want the app gone too, the Applications module takes the app and its leftovers together.

Step four: if you use Docker, look there next

Docker Desktop keeps all its images, containers and volumes inside one virtual disk file, and that file is a frequent answer to "where did 60 GB go?" SweepyMac deliberately never touches Docker images, because deleting them is not something a general cleaner should decide for you.

Use Docker's own tools. docker system df shows what is using the space, and docker system prune removes what is unused after asking. Settings › Resources shows the disk image's location and its limit. Be careful with that limit: Docker's documentation warns that reducing it deletes the current disk image, along with every container and image inside.

Step five: if you use Xcode, clear what it rebuilds

Simulators and device support files can each run to many gigabytes. SweepyMac leaves device support alone, and this is where Stackyard comes in. Its Dev Caches tab measures DerivedData, Device Support and simulators alongside npm, Yarn, pnpm, Cargo and the rest, and Select All Safe ticks what rebuilds by itself. Xcode Archives are never touched, since those cannot be rebuilt without the original source at that commit. There is more in the post on dev caches.

Check your backups for phones

Local iPhone and iPad backups live in ~/Library/Application Support/MobileSync/Backup and can be very large. Depending on your macOS version they may be counted separately from System Data. Do not delete them from that folder. Connect the phone, open it in Finder, and use Manage Backups, so you can see which device each one belongs to.

What to expect

You will rarely get System Data to zero, and it is not a useful goal. macOS needs some of it to run. What you want is a bar you can explain. Once caches, dead app data and old disk images are gone, what remains is mostly what your Mac is using.

SweepyMac has a three-day free trial with no account, then from $14.99 a year: get it here. Developers should also look at Stackyard, free for three days, then from $21 once.