What you can scan
This Mac scans the startup disk, following the firmlinks into the data volume so your home folder, applications and documents are counted. Home scans your home folder alone. Folder scans anything you choose, including an external disk.
- This Mac — ⌘N
- Scan a folder — ⌘O
- Scan the same place again — ⇧⌘R
How long it takes
DiskDuo reads directories in parallel and builds the tree in one pass. On a warm cache it measures well over a million files a minute; a cold cache, a nearly full disk or another program hammering the same disk will slow it down.
Progress is measured against something real: your last scan of the same place, or the space the disk reports as used. Where nothing honest is known, it counts rather than inventing a percentage.
Sizes that tell the truth
Size on disk is what the file system actually allocates. Hard links are counted once. APFS clones are recognised, so a copy that shares its blocks is not counted as space you could reclaim. Compressed files report both what they take and what they hold.
Leaving things out
Settings holds a list of excluded paths, and every way of starting a scan honours it. Turning on “Follow other volumes” lets a scan cross onto other disks; by default it stays on the one you asked for.
Stopping a scan
Cancel stops it and keeps what has been measured so far, marked as partial. A partial scan is never saved as a snapshot and never becomes the basis for the next scan’s percentage.