• sync between desktop and laptop
  • keep a third copy on the Raspberry Pi
  • Pi always on as no fan and only uses 5w.

Syncthing site has an apt page which can be useful, e.g. it had the correct keys for the repo which other instruction pages did not (always getting errors on apt update).

If the repo key is missing:
sudo curl -o /usr/share/keyrings/syncthing-archive-keyring.gpg

Syncthing seems to work fine. Others left below for reference.

or nextcloud maybe…

Open Media Vault also looks easy to install