FileFluss

FileFluss shows a left and a right panel, and each panel can display a local folder or a connected cloud account. You can drag, copy, move, sync or compare between the two, and search every connected account at once. It supports Google Drive, Dropbox, OneDrive, iCloud Drive, S3, WebDAV, SFTP and many other services, can mount cloud accounts in Finder, and can keep an offline index of an account for browsing and search without a connection. It suits people who move files between a Mac and several cloud accounts.

FileFluss requires macOS 14 or later. The DMG is signed with a Developer ID certificate and notarized. The README says the app has no servers, accounts or telemetry; its only automatic request is an update check against GitHub, which can be switched off. Cloud-to-cloud transfers are staged through a local cache folder, so they need free space on the Mac as well as in the account.