Depois de muito custo deu certo!
sudo chown -R pi:pi /extdrive
sudo chmod 777 /extdrive
sudo apt-get install transmission-daemon
sudo usermod -a -G pi debian-transmission
sudo nano /var/lib/transmission-daemon/info/settings.json
download-dir: /extdrive,
incomplete-dir: /extdrive,
incomplete-dir-enabled: true,
rpc-authentication-required: false,
rpc-whitelist-enabled: false,
seed-queue-enabled: true,