This does not work:
->cp seafile-data /tmp/
cp: omitting directory ‘seafile-data’
You need to add the -r (== --recursive) parameter. I prefer to use:
mkdir /mnt/YourMountingnameForExternalVolume/seafile-data
rsync -av --progress seafile-data/ /mnt/YourMountingnameForExternalVolume/seafile-data/