ChronoSync is a Mac app from Econ Technologies, Inc. that can synchronize and backup to almost any storage connected to a Mac, including local files (folder to folder sync), other Macs or PCs, external drives, and cloud services. You can schedule tasks, customize rules/filters, define scripts that can run multiple syncs simultaneously, automatically archive deleted or replaced files, use NFS file servers and more.
ChronoSync 4.9.8 is priced at $49.99 for OS X 10.11 and up with a 15-day trial available. (Older versions are available for earlier Macs.) The latest release addresses several more macOS Catalina issues, among other updates.
ChronoAgent 1.9.6, priced starting at $14.99 per license, “is an invisible application that runs silently in the background on a Mac and provides ChronoSync or InterConneX a direct connection to a remote Mac”, protected by a login and optional encryption. ChronoSync can be used as a backup server, pushing or pulling data as required.
InterConneX 2.0 is a free file management app for iOS 9 and later that can consolidate mobile files in a “sharespace” container and synchronize files with Macs running ChronoSync. Version 2.0 improved robustness (in conjuction with a ChronoSync update) and brought a new “adaptive interface which allows multitasking and support for drag and drop.”
ChronoMonitor is a $3.99 app for iOS 7 and up “that monitors Macs running ChronoSync or ChronoAgent.”