What was found
AT CRUISING ALTITUDE "R GEN DRIVE EICAS MSG DISPLAYED." IAW GEN DRIVE CHECKLIST COMPLIED WITH DISCONNECTING THE DRIVE AND STARTING APU. UPON STARTING THE APU, APU GEN OFF EICAS MSG DISPLAYED. RESET OF APU GEN NOT SUCCESSFUL. QRG DIRECTED LANDING AT NEAREST SUITABLE AIRPORT. DIVERSION TO KDFW WITH AN EMERGENCY DECLARATION. AIRCRAFT GROUNDED. APU START WITH RIGHT GENERATOR SUCCESSFUL. NO FAULTS NOTED ON BPCU. NO PREVIOUS FAULTS RECORDED. SHUTDOWN APU AND RESTARTED AGAIN. NO FAULTS NOTED. RH GEN DEFERRED IAW ATI MEL 24-00-01-01. CAT C NO FLY DATE 01/15/2018. MX PROCEDURES COMPLIED WITH. REMOVED AND REPLACED RH IDG. NO LEAKS NOTED. OPC SATISFACTORY. DMI CLEAR. ACFT RELEASED TO SERVICE.