#TIL of the #SATA pin 3 syndrome. A #Seagate Exos X18 18TB would not power on when connected via SATA power, but would via molex-to-SATA. Seems my #Corsair RM750x doesn't support SATA revision 3.3's "power disable" spec, so I had to mask out pins. Older Exos X18 16TB works fine.