What was found
FLIGHT 2351. "IDG 1 OFF BUS" EICAS MESSAGE IN FLIGHT. RAN CHECKLIST AND TURNED IDG OFF. DIVERTED AND ACCOMPLISHED UNEVENTFUL LANDING. REMOVED FROM SERVICE. INTERROGATED CMC, FOUND FAULT CODE 24210002GU1. PERFORMED FIM 24-21-00-810-803-A AND REPLACED GCU1 IAW AMM TASK 24-21-19-000/400-801-A. OPS CHECK PASSED. PERFORMED ENGINE RUNS WITH CREW AND IDG 1 OR FAILED OPS CHECK REF AMM TASK 24-21-01-710-801-A. "IDG 1 OFF BUS" REMAINED ON EICAS PLACING IDG 1 ON MEL 24-21-0. MEL CLEARED 5/5 BY RECONNECTED IDG 1 IAW AMM 24-21-840-801-A, OPS TEST FAILED. "IDG 1 OFF BUS" REMAINS ON EICAS. REPLACED IDG 1 IAW AMM 24-21-01-000/400-801-A. PERFORMED OPS TEST OF IDG 1, OPS TEST GOOD. "IDG 1 OFF BUS" NO LONGER SHOWS ON EICAS. NFDN. PLACARDS REMOVED, MTX CONTROL NOTIFIED. THIS CLEARS MEL. RETURNED TO SERVICE.