Privacy & safety · Keeping your files safe
What happens if an app is using a file when Storage Studio wants to move it?
Short answer: It lists every app using files in the folder, with Quit, Quit all and Recheck buttons. Quitting is graceful, never forced, so apps can save their work first. System housekeeping services are ignored because they don't interfere.
Storage Studio checks first. The move confirmation lists every app using a file inside the folder, with a Quit button for each, Quit all, and Recheck. Quitting is always graceful — it's never a forced kill, so the app can save its work.
macOS's own housekeeping services, like Spotlight indexing, are left off the list; they can't be quit and don't interfere.
For moving things back, an amber Force move back option lets you proceed anyway — files that exist only on your built-in drive are always kept. See What's Using Space.