How to construct a model

Everyday, we would face many different kinds of problems. Before resolving them, constructing a model for a specific kind of problem is better, cause we can reuse that model.Today,a problem gives me a chance to construct a model, things like this:Before closing my software, I want it to save all of its documents. If everything is ok, I just make it execute saving method.But sometimes, everything is not ok.For example, when my software is opening, I delete its folder which includes all of the opened documents. At this time,when I click the "Close" button to close the currnet project, saving will fail because its documents cannot find its local path.Another example: When I opening a project whick is saved in a ROM CD and closing it, saving will also fail because the driver it belongs to is read only.Everythins is ok and maybe everything is not ok. How to construct a model to resolve this kind of problem?Thx! Any suggesti ...