The magnetization dynamics of individual nanomagnets is studied by spin-polarized scanning tunneling microscopy, combining real-time telegraphic noise analysis with pump-probe schemes. A transition between two Arrhenius regimes is observed as a function of temperature. The switching rates at high temperature are found to be orders of magnitude lower than expected from the extrapolation from the low-temperature regime. A four-state hopping model of magnetization reversal is developed to interpret the experimental results in terms of nucleation, annihilation, and propagation.