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The cooler synchrotron COSY was intended from its inception to provide also deuteron beams which, in addition to protons, could considerably expand the range of experiments accessible at this accelerator facility. A major obstacle to deliver the required D[sup -] beams with the injector cyclotron JULIC has been its electrostatic septum deflector at extraction. After the conversion of JULIC into a negative ion machine, operation of the original deflector at the voltage levels necessary for D[sup -] extraction proved to be impossible with reversed polarity due to severe electrical strength problems like sparking and dark currents. The design and development of a suitable septum deflector turned out to be a challenging and tedious process lasting several years. The new device is in operation since January 2000 and the cyclotron was successfully tested in October for delivering D-beams at 75 MeV with an extracted current in the order of 10 µA. Development, test procedures and characteristics of the new electrostatic septum deflector are reported together with its operational performance to date. Results achieved during the first test run with D[sup -] ions are presented, including injection and acceleration of the deuteron beam in COSY. [ABSTRACT FROM AUTHOR]