Hi, this is potentially not about a temp file because of “Saving config:..” but you can copy your syncthing configuration folder to your external disk and link to it or use one of the command line options at startup to set the config/db directory to the external disk. Command line options can be found here.
EDIT: UPS, I didn’t saw the tmp extension. You may want to look at this thread or this fix and retry with the current version as it may be fixed already.