From https://syncthing.net/

Syncthing is a continuous file synchronization program. It synchronizes files between two or more computers in real time, safely protected from prying eyes. Your data is your data alone and you deserve to choose where it is stored, whether it is shared with some third party, and how it’s transmitted over the internet.

For a given folder on a given machine:

• Folder ID: Unique identifier for the folder across all devices (it's how Syncthing knows two folders are the same)
• Folder Path: The actual location of the folder on this device
• Global State: The combined view of all files in the folder across all connected devices
• Local State: The actual files currently present on this machine
  If Global > Local: this machine is missing files (needs to download)
  If Local > Global: this machine has changes not yet shared (needs to upload)
• Folder Type: Send & Receive (default): this machine sends and receives files
  Send only: this machine sends files, doesn't receive. if it's missing changes, see: "Out of Sync (override changes)"
  Receive only: this machine receives changes, doesn't send any changes made on this machine (if that happens, it's flagged "Local changes")
• Rescans: How often Syncthing checks the folder for changes
• File Pull Order: Determines which files get pulled first when syncing. Choices: Alphabetic, Smallest First Largest First, Oldest First, Newest First, Random
• Shared With: The list of other devices that sync this folder. Each device must be connected (paired) and accept the folder share.
• Last Scan: The last time the folder was scanned.

• Download and Upload Rate: How fast this device is receiving or sending data from/to other devices
• Local State (Total): Total size and file count of all data Syncthing is tracking on this device
• Listeners: Active Listeners / Configured Listeners
• Discovery: Working discovery methods / Configured discovery methods.  If something like 4/5, means one method failed or is unavailable. This is common and usually not a problem (unless you're having connection issues).

Windows machine is named "W" and Mac machine is named "M".  We'll have W backup its Documents and Downloads directory to M' external HDD DE32.

The intention is that M becomes the *mirror* of W’s folders, stored on an external HDD (`/Volumes/DE32`) on M. Here’s a step-by-step plan to do it safely.

• On W (Windows machine)
  1. Install and run Syncthing. 
  2. Identify the folders to back up: 
     1. Downloads: `C:\Users\<User>\Downloads`
     2. Documents: `C:\Users\<User>\Documents`
• On M (Mac machine)
  1. Install and run Syncthing. 
  2. Ensure external HDD (`DE32`) is mounted and has enough space. 
  3. Decide on backup paths on M: 
     1. `/Volumes/DE32/W-Downloads`
     2. `/Volumes/DE32/W-Documents`

• On W: 
  1. Open Syncthing and click Add Folder for each folder if not already shared. 
  2. Set a Folder ID, e.g., `W-Downloads` or `W-Documents`. 
  3. Share each folder with M.

Note: Folder names on W and M do not need to match. Syncthing uses Folder ID to identify folders.

• On M: 
  1. Accept incoming share requests from W for `W-Downloads` and `W-Documents`. 
  2. Set the Folder Path to the external HDD: 
     1. `/Volumes/DE32/W-Downloads`
     2. `/Volumes/DE32/W-Documents`
  3. Set Folder Type → Receive Only to make M a read-only mirror of W.

• Syncthing will start scanning and then syncing files from W to M. 
• Monitor the folder status: `Scanning` → `Out of Sync` → `Up to Date`. 
• For large folders, initial sync may take time.

• To avoid backing up temporary or incomplete files in Downloads, add ignore patterns on W:

*.tmp
*.crdownload
*.part
.DS_Store
Thumbs.db

• Syncthing continuously monitors W for changes and replicates them to M. 
• Keep external HDD mounted. 
• Keep Folder Type Receive Only. 
• Optionally, reduce rescan frequency for large folders to save CPU.

• Do not set M to Send & Receive for these folders. 
• Ensure external HDD is always mounted; otherwise, M will report "Folder path missing." 
• Use clear Folder IDs (`W-Downloads`, `W-Documents`) to avoid confusion.

• W’s Downloads and Documents are continuously mirrored to M’s external HDD. 
• M cannot accidentally overwrite W. 
• Provides an offline-friendly backup in `/Volumes/DE32`.