Syncthing is a continuous file synchronization program. It synchronizes files between two or more computers in real time, safely protected from prying eyes.¹⁾

Official page: http://syncthing.net
Windows download: https://github.com/canton7/SyncTrayzor/releases/ (You want SyncTrayzorSetup-x64.exe)

Syncthing is basically like Dropbox, MEGA, or Google Drive - except that there is no central server, and all synchronization happens between the computers themselves.

Primarily, this means there's no capacity limits (except your HDDs), no big brother spying, no possible shutdown involved. But it also means there is no way to link Syncthing stuff to the outside, and that the computers need to be online at the same time to sync.

Its primary use-case is when a single person has more computers, and wants to keep some folders synchronized on all of them.

What we use it for, however, is general data sharing - there's semi-public Syncthing folders (listed below). It works well for common (audio, video) files and projects, for sharing essential software packages, and for community-powered backups as well. For local sync - there is no need for internet, local network is enough.

https://docs.syncthing.net/intro/getting-started.html

Send / Receive Only are setting folder behaviour on device >

1. All changes from other devices in the cluster are ignored.
2. If it becomes out of sync, a red “Override Changes” button will override any changes made to files through whole cluster will be overwritten by the version on this host, any files that don’t exist on this host will be deleted, and so on.
3. Good as master for backups.

1. Local changes are not distributed to other devices.
2. Good as slave for backups.

You can use syncthing with external harddrives. That is good for backup clusters, huge projects, etc.

On windows it helps to set fixed drive letter to your hardrive, so there are no mismatches.

When assigning your drive letter, make sure to choose a letter toward the end of the alphabet list. For example, X, Y, or Z — otherwise Windows has the tendency to eventually assign a different letter. Also, keep in mind that it will only be the assigned letter on the computer you changed it on. If you take your external drive to a different PC, Windows will give it a different dynamic letter.²⁾

See syncthing