Directed model checking has proved itself to be a useful technique in reducing the state space of the problem graph. But still, its potential is limited by the available memory. This problem can be circumvented by the use of secondary storage devices to store the state space. This paper discusses directed best-first search to enhance error detection capabilities of model checkers like SPIN by using a streamed access to secondary storage. We explain, how to extend SPIN to allow external state access, and how to adapt heuristic search algorithms to ease error detection for this case. We call our derivate IO-HSF-SPIN. In the theoretical part of the paper, we extend the heuristic-based external searching algorithm to general weighted and directed graphs. We conduct experiments with some challenging protocols in Promela syntax like GIOP and dining philosophers and have succeeded in solving some hard instances externally.
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