In the failover configuration make sure that The two units in a failover configuration must have the same hardware configuration. They must be the same model, have the same number and types of interfaces, the same amount of RAM, and, for the ASA 5500 series security appliance, the same SSMs installed (if any).
The two units do not have to have the same size Flash memory. If using units with different Flash memory sizes in your failover configuration, make sure the unit with the smaller Flash memory has enough space to accommodate the software image files and the configuration files. If it does not, configuration synchronization from the unit with the larger Flash memory to the unit with the smaller Flash memory will fail.
