Right-clicking my removable volumes doesn't show scan as an available action, so it's looking more like scanning removable volumes isn't supported at all (also not what I want). Avast auto-detects Time Machine volumes and excludes them-they're grayed / can't be toggled-but all other removables are toggle-able: I located a couple other threads referencing backups, but there's not a lot of definitive info other than Time Machine / backup volumes are bad to scan (very true!) and thus aren't. Thanks for the info, Since there are no toggles/controls, it's not obvious what it's scanning vs not. If the mac version is anything like the Windows version, It does not automatically scan any external drives.