What is System Data on Mac — and how to delete it safely

System Data is a leftover bucket, not a thing

Open System Settings → General → Storage and you get a stacked bar: Applications, Documents, Photos, Mail, macOS, and a grey segment called System Data (it was called "Other" before macOS Ventura). On a fresh install it is 10–20 GB. On a machine that has been worked on for a year it is regularly 60–150 GB, and the panel gives you no way to open it.

The reason it is opaque is that it is defined by exclusion. macOS classifies files it recognises into named categories; everything else — every cache, log, container, virtual disk, simulator runtime, swap file and snapshot — falls into System Data by default. It is not corruption and it is not a bug. It is a label for "unclassified".

That also means "how do I clear System Data" has no single answer. You have to find out which unclassified things are big on your machine, and each of them has its own correct removal procedure.

What is actually in there

In practice the grey segment is dominated by a handful of directories. Ordered by how often they turn out to be the culprit:

• ~/Library/Developer/Xcode/DerivedData — build intermediates. Rebuildable, often 10–60 GB.
• ~/Library/Developer/CoreSimulator — iOS simulator devices and downloaded runtimes. Each runtime is 6–10 GB and old ones are never removed automatically.
• ~/Library/Containers/com.docker.docker — Docker Desktop's virtual disk. It grows and does not shrink on its own.
• ~/Library/Caches and /Library/Caches — app caches. Chrome, Slack, Spotify, Adobe and Xcode are the usual heavyweights.
• Time Machine local snapshots — hourly APFS snapshots of your own drive, reported as purgeable.
• ~/Library/Application Support — real app data mixed with app-managed caches. Treat case by case.
• /private/var/vm — swap and sleepimage. Managed by macOS. Do not touch.
• ~/Library/Mail — downloaded message bodies and attachments.
• ~/Library/Group Containers — shared data for sandboxed apps, including Mail and the iWork suite.

Measure it, then attack the largest item

The Storage panel will not break System Data down, so use something that walks the file system directly. Both of these are read-only:

The first command ranks the top level of your Library folder; the second shows what Time Machine is holding in local snapshots, which the panel counts as purgeable rather than as System Data but which people usually mean when they say "space I cannot see".

du -h -d 1 ~/Library 2>/dev/null | sort -rh | head -20

tmutil listlocalsnapshots /

Shrinking each contributor, safely

Xcode DerivedData can be deleted entirely; Xcode rebuilds it on the next build, at the cost of one slow build. Old simulator runtimes are removed with `xcrun simctl runtime delete` or from Xcode → Settings → Platforms. Both have a dedicated article on this blog.

Docker only returns space when asked. `docker system prune -a` removes unused images, containers and build cache — it is destructive to anything not currently in use, so read the confirmation prompt. On Docker Desktop 4.x the virtual disk file itself is also trimmed from Settings → Resources → Advanced.

Caches can go to the Trash after quitting the owning app. Mail attachments are best shed via Mail → Settings → Accounts → Download Attachments: "None". Local snapshots thin themselves automatically when the drive fills, and can be thinned on demand with `tmutil thinlocalsnapshots / <bytes> 4`.

Restarting the Mac is a genuinely useful last step, not folklore: it clears swap in /private/var/vm, flushes tmp directories, and prompts macOS to recompute the Storage figures, which are cached and often stale after a large deletion.

Deleting files inside ~/Library/Containers by hand can destroy app data — that is where sandboxed apps keep your documents, not just caches. Uninstall the app or use its own settings instead.

Why the number does not drop immediately

You delete 40 GB and System Data barely moves. Three ordinary reasons, in order of likelihood: the files are still in the Trash, which is the same disk; Time Machine took a local snapshot before the deletion, so the blocks are still referenced and will only be released when the snapshot is thinned; or the Storage panel is showing a cached figure. Empty the Trash, wait a few minutes, and reopen the panel.

If space is still not returned after a reboot, check for a snapshot holding it with `tmutil listlocalsnapshots /`. A snapshot taken minutes before a big delete will pin those blocks for up to 24 hours unless you thin it manually.

A worked example

Here is a real shape this takes, because the abstract advice is less useful than one concrete run-through. A 512 GB MacBook Pro reports 61 GB free and 148 GB of System Data. The Storage panel offers nothing further.

Ranking ~/Library shows Developer at 71 GB, Containers at 38 GB and Caches at 9 GB. Inside Developer, DerivedData is 31 GB and CoreSimulator is 34 GB — of which four downloaded runtimes for iOS versions below the app's deployment target account for 27 GB. Inside Containers, com.docker.docker is 32 GB, most of it a virtual disk holding images for a project that finished in March.

The fix is then obvious and takes about ten minutes: delete DerivedData (31 GB, rebuilt on the next build), remove the three oldest simulator runtimes and run `xcrun simctl delete unavailable` (27 GB), and run `docker system prune -a` after checking that nothing in use is affected (24 GB). That is 82 GB, none of it irreplaceable, and none of it discoverable from the Storage panel.

The general lesson is that System Data on any particular Mac is usually two or three large things, not a thousand small ones. Ranking beats sweeping every time.

A view that never needs a System Data category

A directory-tree analyzer sidesteps the whole problem: it does not classify anything, it just shows every folder sized by the bytes underneath it. Dropit scans a full Mac and draws it as a treemap, so what the Storage panel calls "System Data" appears as the specific folders it really is — CoreSimulator here, Docker there — with sizes you can compare at a glance. Scanning and every view are free; the staging-and-deleting half is what you pay for once. Either way you stop guessing at a grey rectangle.