We study the isotropic-nematic transition of hard cut-spheres over a wide range of aspect ratios using Monte Carlo simulation and umbrella sampling. We find a minimum in the density of both coexisting phases near to L/D = 0.04 (thickness/diameter) and show that this minimum is a result of competition between the solid volume of the disks and the free volume which is significantly influenced by nematic pre-ordering in the isotropic phase. We expect similar behaviour for other kinds of discotic particles. This behaviour should be observed as a kind of 'partially re-entrant' isotropic phase for discotic mesogens within an appropriate concentration range. (C) 2009 Elsevier B. V. All rights reserved.