System Data on a Mac is usually caches, logs, Time Machine local snapshots, old iOS backups, and leftovers from deleted apps. Most of it is safe to clear, and on a 1TB Mac it's often 100GB or more. macOS shows you the gray bar in System Settings, then gives you no button to do anything about it. The fixes below go from the quickest win to the most thorough, with the exact commands for each.
TL;DR: Check Time Machine local snapshots first (tmutil listlocalsnapshots /), they're often tens of GB. Then clear app caches, old iOS backups, and developer junk, or let Mole (free, open source) do all of it in one pass. In our test that took free space from 183GB to 377GB. If the space eater is footage or photo libraries rather than System Data, that's a backup workflow problem, covered at the end.
What Is System Data on Mac?
Open System Settings → General → Storage and you'll see it: a gray "System Data" segment that on many Macs is the single largest category. It's everything macOS can't sort into a named bucket like Apps or Photos. In practice that means app caches, system and app logs, temporary files that never got cleaned up, Time Machine local snapshots, old iPhone backups, virtual machine disks, and files left behind by apps you deleted months ago.
A healthy System Data figure is roughly 10 to 30GB. If yours shows 80, 120, 200GB, something specific is eating the space, and you can find and remove it. Take the fixes in order; each one tells you how to check whether it applies to you.
Fix 1: Delete Time Machine Local Snapshots
When Time Machine can't reach its backup drive, macOS quietly keeps hourly snapshots on your internal disk instead. They live inside System Data and can occupy tens of gigabytes. Check whether you have any:
tmutil listlocalsnapshots /
Each line like com.apple.TimeMachine.2026-08-01-120000.local is one snapshot. Delete one by its date string:
sudo tmutil deletelocalsnapshots 2026-08-01-120000
macOS marks snapshot space as "purgeable" and reclaims it when needed, which is why the Storage panel sometimes doesn't update immediately. The snapshots are gone regardless. If you use Time Machine, plug the backup drive in and let it complete a backup; completed backups let macOS thin the local snapshots on its own.
Fix 2: Clear App Caches
App caches are the classic System Data filler. See which apps are hoarding:
du -sh ~/Library/Caches/* | sort -rh | head -20
The top entries are usually browsers, Spotify, Slack, and anything Electron-based. You can delete the contents of individual folders in ~/Library/Caches/ safely; apps rebuild what they need on next launch. Don't delete the Caches folder itself, and skip anything you don't recognize inside /Library/Caches (the system-level one) unless you know what it belongs to.
Fix 3: Remove Old iOS Backups
If you've ever backed up an iPhone or iPad to this Mac, those backups are still there, and each one can be 5 to 50GB. In Finder, connect nothing, and go to ~/Library/Application Support/MobileSync/Backup/, or check via System Settings → General → Storage → iOS Files. Delete backups for devices you no longer own or that now back up to iCloud.
Fix 4: Delete Developer Junk (If You Use Xcode)
Xcode's DerivedData folder grows without limit and is always safe to delete:
rm -rf ~/Library/Developer/Xcode/DerivedData
Old iOS simulator runtimes are the other big one; remove unused simulators in Xcode → Settings → Platforms. Skip this fix entirely if you don't build software.
Fix 5: Restart Your Mac
Not a joke fix. A restart clears /private/var/tmp, sleeps images, and lets macOS actually reclaim purgeable space that the Storage panel has been counting against you. Restart after doing fixes 1 to 4, then re-check the Storage panel before assuming something's still wrong.
Fix 6: Run Mole and Do All of It in One Pass
Mole is a free, open source terminal tool by tw93 (GPL-3.0, 57,000+ stars on GitHub) that scans all of the junk categories above and deletes what nothing uses anymore. Mole is a third-party project with no connection to Tusk. For this specific problem it's the best tool we've found.
brew install mole # or see the install script on the GitHub page
mo clean --dry-run # preview what would be deleted, touches nothing
mo clean # actually clean
If you're cautious, run the dry-run first. It lists everything Mole would remove before anything happens. In our test on a 1TB MacBook Pro with 155GB of System Data, one mo clean took free space from 183GB to 377GB.
The full process, including the Mole demo and the footage workflow below, is also on video:
Fix 7: When It's Not System Data, It's Your Files
Clearing System Data buys space back once. Caches grow back, snapshots accumulate again, and if your Mac refills every few weeks, the recurring weight usually isn't System Data at all. Check the other bars in the Storage panel: on most full Macs it's video footage, photo libraries, or project files doing the real damage.
That has a different fix, because you can't just delete your own work. What you can do is stop using your Mac as the warehouse.
Tusk is a Mac app that automatically backs up your project folders to multiple locations and tracks where every file lives — even when your drives aren't connected.
The workflow shift: your files live on external drives or in the cloud, verified as backed up, and your Mac holds only what you're actively working on. The reason most people don't work this way isn't discipline. It's that deleting local files feels risky when you can't confirm the backup is complete. Not sure if the drive was plugged in when the sync ran. Not sure if every file made it across. So everything stays, and you buy another drive.
Tusk exists for exactly that gap. You point it at a project folder and add backup destinations (external drives, S3-compatible cloud, Google Drive, or several at once). It backs up every change automatically and verifies each file with a BLAKE3 checksum. When you want space back, Tusk shows you which files are confirmed on every destination, and those come off your Mac in one click. Files that aren't fully covered stay put; Tusk refuses to delete them. Months later it still knows which drive holds what, even with every drive unplugged.
If that's your situation, the 3-2-1 backup guide for video projects covers the full setup, and the storage sizing guide helps you work out how much backup capacity the workflow actually needs.
FAQ
System Data is the gray category in macOS Storage settings that holds app caches, logs, Time Machine local snapshots, old iOS backups, and leftovers from deleted apps. Most of it is safe to clear: caches rebuild themselves, snapshots are only useful until Time Machine completes a real backup, and old iOS backups for devices you no longer own do nothing. What you should not do is delete unfamiliar files inside /System or /Library/Caches at the system level without knowing what they belong to.
The usual culprits, in order of size: Time Machine local snapshots that accumulated while your backup drive was unplugged (check with tmutil listlocalsnapshots /), app caches from browsers and Electron apps like Slack or Spotify, old iPhone/iPad backups in MobileSync, and developer files like Xcode's DerivedData. A normal System Data figure is roughly 10-30GB; anything approaching 100GB means one of these categories has grown unchecked.
Four manual steps cover most of it: delete Time Machine local snapshots (sudo tmutil deletelocalsnapshots followed by the snapshot's date string), clear per-app folders inside ~/Library/Caches, remove old iOS backups under System Settings → General → Storage → iOS Files, and restart so macOS reclaims purgeable space. That's the same territory a cleaning tool covers, done by hand.
Roughly 10-30GB on a typical Mac. Developers and heavy creative-app users run higher legitimately (simulator runtimes, render caches). If System Data exceeds 80-100GB, it almost always contains something removable: local snapshots, stale caches, or old device backups.
The key is having a confirmed backup before you delete local copies. A tool that tracks backup status per file will tell you exactly which footage is safely stored on your external drives or in the cloud before you remove it from your Mac. Once a file is verified across your backup destinations, you can delete the local copy without losing the footage. It's still on your drives.
Cloud can work as one layer of a backup strategy, but it's usually not practical as the primary way to free up local space for large footage files. Uploading hundreds of gigabytes takes time and ongoing cloud storage costs add up. Most photographers and videographers use local drives as the primary backup and cloud as a secondary layer. Tools like Tusk support both at the same time, so you can back up to drives and to S3-compatible cloud storage simultaneously.