So you have G4XX Media Gateways only?
You could disable the Processor Ethernet on the Main server.
From SAT on the MAIN server run:
change ip-interface procr
Then change the y to a n:
Enable Interface? n
***WARNING*** this WILL interrupt service.
Then from the other ESS that you already know works, run the same command.
You should also log into each of your media gateways and copy off the configuration. Check the MGC List,Reset-Times and Transition-Points to make sure they are set the way you want them.
Also on the SAT of the Main server, check the Recovery Rule for each media gateway and run change system-parameters mg-recovery-rule X (where X is the assigned recovery rule number). Make sure the recovery rule is as expected.