Recent work has shown that the durability of large-scale storage systems such as DHTs can be predicted using a Markov chain model. However, accurate predictions are only possible if the model parameters are also estimated accurately. We show that the Markov chain rates proposed by other authors do not consider several aspects of the system’s behavior, and produce unrealistic predictions. We present a new analytical expression for the chain rates that is condiderably more fine-grain that previous estimations. Our experiments show that the loss rate predicted by our model is much more accurate than previous estimations.