Bem, você só pode redimensionar volumes em discos que usam a tabela de partição GUID, não discos MBR.
Consulte Suporte da Apple :
Mac OS X v10.5 Leopard can resize any disk partition except a Master Boot Record Partition. You can use Disk Utility to see if a partition is a Master Boot Record Partition, and also to resize eligible partitions.
Important: Resizing a Master Boot Record partition will erase all data on it, so be sure to back up important files on affected partitions first.