Duplicate files · Where it looks and what it skips
Does the duplicate finder check my external drive?
Short answer: No. The duplicate finder checks Downloads, Documents, Desktop, Movies and Pictures on your Mac, plus iCloud Drive and connected cloud accounts. External drives aren't scanned, and folders Storage Studio has moved to your drive are left out.
No. The This Mac source checks folders in your home folder — Downloads, Documents, Desktop, Movies and Pictures — and external drives aren't scanned. Folders Storage Studio has moved to your drive for you are also left out, because it manages those itself.
The sources are also checked one at a time, so a file on your Mac and a copy of it somewhere else (another drive, or a cloud account) aren't compared against each other.
If your goal is to free space on your Mac rather than on the drive, moving big folders is often the bigger win — see moving folders.