What a scan leaves out

A scan of a home folder walks past hundreds of thousands of files it should not open. What it steps over, and why each one is worth stepping over.

Other applications' storage

The walk never enters Library/Containers or Library/Group Containers. Those hold the private storage of every other app on the machine — their own data, not your pictures or documents.

The reason is not only that there is nothing in there worth having. macOS guards each container separately, and reading one from outside does not fail cleanly: the read waits for an answer that never comes. On a normal machine that is 956 containers and 194 group containers, all behaving the same way, and a scan of a home directory had every worker stuck within four seconds.

Nothing you can reach is lost by this. The two locations the app offers by name, Library/Mail and Library/Messages, are not containers, and a folder you choose yourself is the starting point of a walk and is never tested against this rule — so you can scan a container if you deliberately point the app at one.

Inside a photo library

A Photos, iPhoto or Aperture library is a folder that keeps your imported files and everything built from them side by side. AssetScout reads the imported originals and steps over the derived data: the previews and thumbnails, the rendered result of an edit, the proxy movies used while editing, the caches of the background analysis, and the database itself. All of it is rebuilt by the app that owns the library if it ever goes missing.

This pays twice. The walk drops a whole subtree in one test rather than looking at every file in it, and one library holds tens of thousands of such files. The second step of the scan then no longer opens files it cannot use: before this rule, a scan of a home directory had to throw away 1 866 files after opening them, and every name in that list was a preview, a sticker preview or a placeholder.

One directory inside a library is deliberately not skipped: the iCloud sync area. A file waiting to be uploaded, or just downloaded, can sit there before it reaches the originals, and it is a real file of yours.

Files nothing could name

A file whose type macOS cannot determine is left out. Previews, caches, and files with a wrong or missing extension all end up here. On a home folder this is thousands of files as a matter of course, which is why the app does not put a message on screen about it.

When a file you care about is missing, the list is there to read: View > Show Skipped Files…. The menu item is greyed out when the last scan left nothing out.

Folders that could not be read at all

A folder inside the scan may refuse to open — usually one belonging to the system or to another user account. The rest of the scan is unaffected, and the app says which folders it happened to. See Messages, and what they mean.