The discovery of new transition metal (TM) oxide cathodes which can act as intercalation hosts for Mg$^{2+}$ ions is critical for the development of high energy density Mg-ion batteries. In Li-ion batteries, disordered rocksalts with sufficiently high Li$^+$ charge carrier ion concentration, i.e. Li : TM >1.1, can
support fast Li$^{+}$ diffusion and therefore deliver high capacities (∼300 mA h g$^{-1}$) and rate performance. Here, we investigate a range of simple Mg-rich disordered rocksalt cathodes, Mg$_2$TMO$_{3}$ (TM = Mn, Ni, Co), which possess similar charge carrier ion concentrations and similar ratios between the ion size and interstitial tetrahedron height to Li-rich disordered rocksalts. However, even with high carbon loadings, elevated cycling temperatures and reduced particle and crystallite size, no significant Mg$^{2+}$ deintercalation was observed, indicating that conventional design rules established for Li-rich DRS cannot be simply translated to Mg-rich systems. Despite the lack of activity in Mg-rich oxides, we demonstrate that Mg$^{2+}$ intercalation into close-packed cubic disordered rocksalts, such as Li $_{2-x}$MnO$_{2}$F (x
= 1), is possible, opening possible routes to activating Mg-rich systems.