Maintaining computer system operability

Disclosed is a method of maintaining the operability of a network-connected data processing apparatus, that is coupled to a host computer, and handles operations on behalf of the host computer. The data processing system has first and second resources to provide redundancy. The host has a failover capability for responding to loss of availability of a first resource by performing failover to the second resource. The host in response to a request for failover from a first resource to a second resource, tests the availability of the second resource for handling operations on its behalf. In response to a positive determination of the availability of the second resource, initiating the failover to the second resource, and in response to a negative determination of the availability of the second resource, providing an indication of unavailability. The failover may be determined as part of a request to take a resource out of service to carry out a resource update.