Duplicate files · Scanning and Auto-scan
What does Scan now do in the Duplicates window?
Short answer: Scan now checks the selected source — This Mac, iCloud Drive or one cloud account — immediately, and turns into Stop while it runs. It only affects that source; each account has its own scan, progress and results.
Scan now starts a check of the source you're looking at — This Mac, iCloud Drive, or one cloud account — straight away, without waiting for the background schedule. While it runs, the button becomes Stop, and a progress line shows what's happening, such as "Scanning your files… found" and then "Comparing files… of …".
Scan now and Stop only affect the selected source. Each cloud account has its own scan, progress and results, so starting one never holds up another, and you can switch between sources while they work.
When it finishes, the status line shows how many duplicate sets were found, how much space can be freed, how many files were compared, and when the scan ran.