macOS keeps some folders private until you allow an app to read them. Full Disk Access lets Wombit measure all of them. It's optional: the app works without it and tells you what it couldn't see.
What the app does with it
The scan reads names, sizes and dates, never your documents. To name a few things, it reads small files apps keep about their own data, such as the device name in an iPhone backup or the version of a macOS installer. Nothing about your files leaves your Mac.
What the scan leaves out without it
Without Full Disk Access, the scan skips Desktop, Documents, Downloads, iCloud Drive and other cloud storage folders. macOS would otherwise stop the scan to ask about each one. It also can't read the private data of Mail, Messages and some other apps.
Nothing is guessed. The Overview shows a banner; its Show Folders button opens Folders the Scan Didn't Read, which lists every folder under Couldn't Read or Skipped on Purpose, so you know which numbers are incomplete.
Allowing it
- When you first open the app, it explains why it asks. Click Open System Settings.
- In System Settings › Privacy & Security › Full Disk Access, drag the app's icon from the app's window into the list, or click + and choose the app. Make sure its switch is on.
- Come back to the app. It notices by itself, though macOS may ask you to quit and reopen it first.
You can skip this and allow it later from the same place in System Settings.
What else it affects
- The menu bar item shares the app's Full Disk Access for its daily measurement. Without it, the measurement skips what it can't read and never asks a second time. See The menu bar and alerts.
- The Trash. Moving items to the Trash, Undo and Put Back work without it. Measuring the whole Trash and emptying it need it, because macOS doesn't let an app list the Trash otherwise.
- Some apps' data. On macOS 27, a few browsers' data folders can only be changed with Full Disk Access. Without it, a cleanup lists those items as Not allowed by macOS and leaves them alone.
Removing it
In System Settings › Privacy & Security › Full Disk Access, turn the app's switch off, or select it and click –. The app keeps working, and the next scan skips the private folders again.