Bytesweep Mac storage full, but nothing is there?
macOS puts most of the answer in a grey bar called "System Data" and gives you no way to open it. There is nothing mysterious inside — it is ten ordinary folders, and you can look at every one of them.
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"System Data" is a leftover, not a category
macOS sorts what it can into Applications, Documents, Photos, Music and so on. Everything it cannot classify falls into one bucket. Older versions called it "Other". It is not a system folder, it is not protected, and much of it is not the operating system at all — on a developer's Mac most of it is your own tooling.
Why the numbers never agree
Three things make the Finder's figure differ from what you can actually free. Purgeable space is counted as used but macOS considers it reclaimable, so it will not let you delete it directly. Online-only iCloud files are counted at full size even though they occupy nothing. And APFS compression and hard links mean a file's logical size and its size on disk are different numbers — the Finder shows you one, du shows you the other.
Check the real figure first
Before you delete anything, see what the disk itself reports, including purgeable:
diskutil info / | grep -E "Free|Available"
df -h /What is actually inside it
Ten items, in the order they usually rank by size. Every one is a real folder you can open.
Time Machine local snapshots
Hourly snapshots kept on the internal drive, even when Time Machine is switched off. Usually the biggest single slice and the hardest to find, because the Finder reports them as purgeable rather than as files.
tmutil listlocalsnapshots /Xcode DerivedData, archives and device support
Build intermediates and symbol caches. Twenty to sixty gigabytes on a working developer's Mac is ordinary.
~/Library/Developer/Xcode/DerivedData
~/Library/Developer/Xcode/iOS DeviceSupportiOS simulator runtimes
A full OS image per simulated iOS version, plus devices Xcode can no longer use.
~/Library/Developer/CoreSimulator/DevicesiPhone and iPad backups
One dated folder per backup, often several for a phone you no longer own.
~/Library/Application Support/MobileSync/BackupDocker's disk image
A single file holding every image, container and volume, which grows and never shrinks by itself.
~/Library/Containers/com.docker.docker/Data/vms/0/data/Docker.rawOr see all ten with real numbers next to them
The hard part is not the list — it is measuring them. Each of these is a different command, and half report a size that disagrees with the Finder. Bytesweep opens the grey bar in about ten seconds, thins Time Machine snapshots with one click, and puts everything you sweep into a restore point you can undo for seven days.
Application caches and containers
Safari, Chrome, Spotify, Slack and every sandboxed app. Individually small, collectively large, and almost all of it regenerates.
~/Library/Caches
~/Library/ContainersMail attachments
Years of downloaded attachments cached locally by Mail.
~/Library/Containers/com.apple.mail/Data/Library/Mail DownloadsLeftovers from deleted applications
Support files, preferences and launch agents from apps that left the Mac years ago. Nothing on macOS ever removes these.
iCloud files that are not really there
Online-only files counted as if they were local. This is the reason "System Data" sometimes shrinks by itself and sometimes will not budge.
Developer package caches
Homebrew, pip, npm, cargo and Gradle each keep an archive cache that rebuilds on demand.
brew cleanup -sSafe, careful, and do not touch
Safe to clear
Application caches, build outputs, package manager caches, unavailable simulators, and Time Machine local snapshots older than your most recent backup. All of these either regenerate or exist elsewhere.
Worth checking first
Device backups, simulator runtimes you might still target, model weights, and archived Xcode builds. Re-obtainable, but slowly — and an archive you delete takes your ability to symbolicate that release's crash reports with it.
Never
Anything in /System, /private/var/db, Keychains, or a database an app currently has open. On a modern Mac the system volume is read-only and signed, so most of this is refused for you — but ~/Library is not protected, and that is where the damage happens.
See the grey bar broken into folders
Bytesweep names every item above, measures it, and thins Time Machine snapshots with one click. Everything you sweep goes to a restore point first, so undo is one click for seven days.
Related: how to free up space on a Mac · disk space analyzer
Questions
Can I just delete System Data?
There is nothing to delete — it is a label on a bar chart, not a folder. You delete the individual things inside it, and some of those are files your apps need. That is why the answer is always "open it and look", never "clear it".
Why is System Data 150 GB on my Mac?
On a developer's Mac the usual answer is Xcode plus Docker plus local snapshots, which together reach that easily. On a Mac used for photos and mail, it is more often local snapshots plus Mail attachments plus iPhone backups.
Why does it grow back after I delete things?
Because most of what is inside is a cache. Clearing Xcode's DerivedData buys you 40 GB until the next build. Local snapshots return within the hour. That is not a fault — it is what those folders are for. What matters is keeping the total in a range you can live with.
Does reinstalling macOS fix it?
It clears it, at the cost of a day. Everything on this page can be cleared without reinstalling, and reinstalling will not stop it coming back, because the same tools rebuild the same caches.
Is purgeable space really free?
Mostly. macOS releases it when something needs the room, so a large copy will usually succeed even when the Finder says you have no space. It will not help you if a tool refuses to start because it checks free space up front, and you cannot release it on demand except by removing the snapshots yourself.