I couple of days ago I posted about ACS and failover. As I wrote there is no failover back to the first ACS collector. If your ACS forwarder failover to a second ACS collector your will have your ACS forwarder data in two databases, as there is a one-to-one relationship between ACS databases and ACS collectors. That will lead to a lot of extra work for you.
I have wrote a simple script to show you one way to get the ACS forwarder to return to the first ACS collector in the list. This script gets the ACS collector settings from the registry, then it test if it can ping the first ACS collector, and if it can, it will restart the ACS forwarder service. That will make the ACS forwarder return to the first ACS collector. If it cant ping the server, it will wait some time and then try again.
Even if your ACS forwarder will start send data to the second ACS collector as soon as your first ACS collector is back online your ACS forwarder will return and you will hopefully not have that much data in the second database.
You can setup a rule to trigger on event ID 4368 from Source AdtAgent in the Application log. The description should also include the name of your second ACS collector. Make the rule run the script as a response.
You can download the script here