You open About This Mac, see 80 GB of "System Data," and wonder where it all came from. For many Mac users in 2026, a surprising chunk of that opaque category is cloud storage system data — sync caches, local copies, and metadata files left behind by Dropbox, Google Drive, and OneDrive that macOS quietly bundles into System Data rather than listing them separately.
What Does macOS Actually Mean by "System Data"?
The System Data category in macOS Storage (Settings → General → Storage) is a catch-all for anything the operating system does not classify as Applications, Documents, Photos, or Media. That includes:
- System and app caches in
~/Library/Cachesand/Library/Caches - Log files in
~/Library/Logsand/var/log - Time Machine local snapshots stored on-disk
- Virtual memory swap files in
/private/var/vm - Cloud sync daemon caches and block stores
Apple does not break this down further in the built-in UI. So when Dropbox stores a 12 GB local cache of recently-accessed files, macOS sees it as "other system-level data" and adds it to the System Data total — invisibly.
Why Cloud Apps Create Such Large Caches
Cloud storage clients are not simple sync tools. They maintain local databases, block-level chunk stores, thumbnail caches, and conflict logs. Here is what each major service typically stores on disk:
| Service | Primary cache location | What it stores |
|---|---|---|
| Dropbox | ~/.dropbox/ and ~/Library/Caches/com.dropbox.DropboxMacApp/ |
Block cache, icon overlays, local DB, thumbnails |
| Google Drive | ~/Library/Application Support/Google/DriveFS/ |
Inode database, file metadata, content cache |
| OneDrive | ~/Library/Containers/com.microsoft.OneDrive-mac/ |
Sync engine database, thumbnail cache, upload queue |
| iCloud Drive | ~/Library/CloudStorage/ and ~/Library/Mobile Documents/ |
Locally-downloaded copies of cloud files, eviction metadata |
All of these locations fall outside the Documents or Applications buckets that macOS tracks, so they accumulate silently inside System Data.
How to See the Hidden Sync Caches on Your Mac
There are two approaches: Terminal commands for a precise read, or a dedicated disk tool that makes the breakdown visual.
Option 1: Check with Terminal
Open Terminal (Applications → Utilities → Terminal) and run these commands one at a time. They report disk usage for the most common sync cache locations.
# Dropbox block cache
du -sh ~/.dropbox 2>/dev/null
du -sh ~/Library/Caches/com.dropbox.DropboxMacApp 2>/dev/null
# Google Drive content cache
du -sh ~/Library/Application\ Support/Google/DriveFS 2>/dev/null
# OneDrive container
du -sh ~/Library/Containers/com.microsoft.OneDrive-mac 2>/dev/null
# iCloud local copies
du -sh ~/Library/Mobile\ Documents 2>/dev/null
du -sh ~/Library/CloudStorage 2>/dev/nullIf any of those numbers surprise you, read on before deleting anything.
Option 2: Use a Disk Map Tool
Crumb includes a Visualize mode that produces a whole-Mac disk map broken down by named category, including sync engine caches. Instead of a single opaque System Data number, you see each cloud app's footprint labeled by name — which makes it far easier to decide what to address. The "Is this safe to delete?" AI feature can also explain any unfamiliar folder before you touch it.
Which Parts of a Sync Cache Are Safe to Delete?
This is where it pays to be careful. Cloud sync caches are not all equal.
- Thumbnail caches — Generally safe. They rebuild automatically the next time you browse files in the cloud app. Located in subdirectories named
thumbnailsorthumbswithin the cache folders listed above. - Content block caches — Safe to delete if the canonical copy lives in the cloud and you have a reliable internet connection. The next access re-downloads the file. On a slow or metered connection this can be painful.
- Sync engine databases — Not safe to delete manually. Files like
sync_engine.db,catalog, or any SQLite file inside the Application Support folder are internal state for the sync daemon. Deleting them can cause the client to re-index your entire cloud library or, in rare cases, trigger spurious conflict copies. - Upload queues — Do not delete while the client is running. These contain files mid-upload that have not yet reached the cloud.
The safest general rule: let the cloud app itself manage its own cache through its preferences. Most clients (Dropbox, OneDrive, Google Drive) expose a "Clear Cache" or "Reset" option in their settings that handles this cleanly and restarts the daemon in a known-good state.
The Safest Way to Reclaim Space from Cloud Sync Caches
- Pause syncing first. In each cloud app's menu-bar icon, pause or quit the sync daemon before touching any files.
- Use the app's own clear-cache option. Dropbox: Preferences → Account → Unlink This Mac, then relink (nuclear option but thorough). Google Drive: quit the app, delete
~/Library/Application Support/Google/DriveFS, reopen Google Drive and let it re-index. OneDrive: Reset from the OneDrive menu. - Remove evicted iCloud files you no longer need. In Finder, right-click any iCloud file marked with a cloud icon and choose "Remove Download." This frees local space while keeping the file in iCloud.
- Consider selective sync. All major cloud clients let you choose which folders sync locally. Turning off local sync for large archive folders you rarely access is the most durable fix — it prevents the cache from growing back.
- Run a system cache clean after. App logs and metadata fragments accumulate even after you clear the cloud cache. A one-click clean pass (Crumb's Clean tab, or
sudo periodic daily weekly monthlyin Terminal) handles system-level leftovers.
What About iCloud's "Purgeable" Space?
macOS has a separate concept called purgeable space — locally-cached iCloud content that the OS has marked as safe to evict automatically when disk pressure rises. This shows up in Disk Utility as light-colored space at the end of the bar. You do not need to manually delete purgeable files; macOS reclaims them on demand. However, they do inflate the System Data number you see in Storage settings, which confuses many users who think they have less free space than they actually do.
If you want to force-reclaim purgeable space immediately rather than waiting for macOS to decide, download Crumb — its Clean tab includes a dedicated purgeable-space pass that triggers the eviction right away.
Preventing the Problem Going Forward
- Enable "Optimize Mac Storage" in iCloud Drive settings (System Settings → Apple ID → iCloud → iCloud Drive). This keeps only recently-used files local.
- Set Dropbox to "Online-Only" mode for folders you rarely open locally.
- Review the Google Drive "Mirror Files" vs "Stream Files" setting. Stream Files mode keeps most content cloud-only and dramatically reduces local cache size.
- Schedule a periodic review of
~/Library/Caches— not to delete blindly, but to spot any single cache folder that has grown unexpectedly large.
The Bottom Line
Cloud storage System Data bloat on Mac is real but manageable once you know where to look. The core issue is that macOS Storage reporting bundles every sync daemon's cache into a single opaque number. Using Terminal commands or a disk-map tool to surface these by name turns a mystery into a list of specific, addressable folders. Go after thumbnail caches and purgeable space first — they are the safest wins. Leave sync-engine databases alone, let each cloud app manage its own reset, and use selective sync to keep the problem from growing back.