We propose a resistivity-beta driving mechanism aimed at explaining the appearance of free boundary long wavelength global instabilities in high-beta diverted tokamaks. These perturbations resemble very closely the RWM (resistive wall mode) phenomenon. Performing a proper toroidal analysis, we show that the magnetohydrodynamic stability is worsened by the interplay of plasma beta and resistivity. By modelling the separatrix through a careful positioning of the resonant surfaces, we find that in an ideal plasma wall effects are effectively screened, so that the ideal beta limit becomes independent of the wall position/physics. A lower wall dependent critical beta is found if plasma resistivity is allowed. We find that global stability can be improved with a toroidal ow, small enough not to induce equilibrium modification. The rotation stabilisation effectiveness depends upon the proximity of the plasma equilibrium parameters to the resistive marginal boundary.