An accessory drive system (20) for a gas turbine engine (10) includes a gearbox (30) driving a plurality of accessories (32) including a cabin air compressor (34) to provide cabin air and an engine starter (36) to start the gas turbine engine. An electrical power supply (50) separate from a main electrical power supply of the gas turbine engine (10) is electrically coupled to the accessories (32). A clutch (60) selectively engages an output (13) of the gas turbine engine (10) to mechanically couple the output (13) to the gearbox (30). The clutch (60) operates between an electrical drive state where it disengages from the output (13) and only the electrical power supply (50) provides electrical power to the cabin air compressor (34) and the engine starter (36), and a mechanical drive state where the clutch (60) engages the output (13) and the gearbox (30) drives at least one of the accessories (32).