From NAS-Central Buffalo - The Linkstation Wiki
Serial @ Pro Duo
Serial Port Pinout
The connector for the Serial port is CN10 it is easy to make out from the frontside image where GND is.
Frontside and Backside Images
The pinout (if you look on the board from the frontside): [RxD]-[TxD]-[Vcc]-[GND]
The pinout (if you look on the board from the backside): [GND]-[Vcc]-[TxD]-[RxD]