The storage bar
System Settings → General → Storage shows a bar divided into categories. Apple defines them in its Storage settings guide; the ones below cause the most questions.
- System Data: Apple’s general category for files that do not belong to any more specific category, such as caches, logs, app containers and developer files. It is a category, not a folder. See the What is System Data guide.
- Other: the name older macOS versions used for roughly the same catch-all. Searches for “Other storage” and “System Data” describe the same problem.
- Available: the space macOS is prepared to give you, which can include purgeable space it has not released yet. Free space is the part already released.
- Used: everything on the volume, including files in the Trash until it is emptied.
Space that comes back on its own
Some of what looks used is space macOS manages for you. Counting it as removable is the most common cleanup mistake.
- Purgeable space: files macOS can remove when it needs the room, as Disk Utility describes. You cannot empty it by hand and it is already inside the available figure. See the purgeable space guide.
- Local snapshot: an hourly copy of the startup disk that Time Machine keeps for 24 hours, plus one before a macOS update. Apple counts its space as available and thins it automatically. See the snapshots guide.
- Cache: working files an app can rebuild. Safe to remove once the app is closed, and expected to come back.
- Optimize Storage: a family of settings that keep originals in iCloud or on the server and smaller copies on the Mac. Photos, Mail and TV each have their own version.
Where files live
Finder hides or bundles several of the places that hold the most space.
- Library: the folder that holds app data. There are three: ~/Library for your account, /Library for all users, and /System/Library for macOS, which is protected. See the Library folder guide.
- Container: a sandboxed app’s private folder inside ~/Library/Containers, including the data for Mail and Messages. Change it through the app, not Finder.
- Package: a folder that Finder shows as a single file, such as the Photos Library or an app bundle. Never edit inside one to save space.
- Disk image: a .dmg file that mounts as a volume. The app you installed was copied out of it, so the image is a leftover once ejected.
- Trash: one per disk. Items deleted from an external drive stay in that drive’s Trash and use its space until you empty the Trash with the drive connected.
Sizes and permissions
Two tools can report different numbers for the same folder and both be right.
- Allocated size: the space a file actually occupies on disk. On APFS, cloned and sparse files can occupy far less than their logical size, which is the size Finder shows first.
- Full Disk Access: the permission under System Settings → Privacy & Security that lets an app read folders macOS otherwise keeps private. Without it, a scanner sees less and should say so.
- Hidden folder: a folder whose name begins with a dot, such as ~/.npm. Finder hides them; Terminal and scanners with Full Disk Access do not. See the Terminal guide.
Cloud and developer terms
These come up whenever a Mac belongs to a developer or syncs with a cloud drive.
- Online-only, streamed or placeholder file: a file that lives in the cloud and takes no local space until opened. The opposite is mirrored or downloaded. See the cloud drive and iCloud guides.
- node_modules: a per-project folder of installed JavaScript dependencies that can be recreated by reinstalling. See the node_modules guide.
- Derived Data: Xcode’s rebuildable build output under ~/Library/Developer. See the Xcode guide.
- Docker.raw: the single file that holds every Docker container, image and volume on a Mac; its allocated size grows and rarely shrinks on its own. See the Docker guide.
- Package cache: downloads kept by npm, pnpm, Yarn, pip or Homebrew so a reinstall is faster. Each manager documents a command to clear it. See the cache guides.
References & further reading
These guides combine the linked official guidance with ClearDisk’s documented behavior. Your files and app settings determine the right next step.