This paper deals with the correlation between the real structure of Cu2Mo6S8 chevrel phase (CuCP) and its electrochemical behavior as a cathode material in novel, promising rechargeable Mg batteries. To obtain fast kinetics of Mg insertion into this material, its particle size was reduced to sub-micrometer scale by milling. We discovered an unusual instability of CuCP upon milling, which leads to a drastic decrease in the electrode capacity. The mechanism of this mechanical instability was investigated by XRD and XPS. The comparison of the CuCP mechanical stability with that of other chevrel phases (Ni, Sn, Pb, or Ag instead of Cu) reveals that the unusual behavior of CuCP is connected to the unstable oxidation state of the copper ions in this material. It was shown that milling of CuCP leads not only to an unusually deep amorphization of the material but also to a chemical reaction in which the Cu+ ions are reduced and removed from the CuCP lattice.