A milling machine for milling a surface of a roadway includes a frame and a milling assembly having a cutter drum that is mounted for rotation with respect to the frame. The cutter drum has a drive shaft on which a sheave is mounted. A primary drum drive assembly is adapted to rotate the cutter drum with respect to the frame. The primary drum drive assembly includes a drive belt that cooperates with the sheave on an input drive shaft for the cutter drum to rotate the cutter drum. A belt lift assembly may be operated to selectively disengage the drive belt from the sheave of the input drive shaft for the cutter drum, so that an auxiliary drum drive assembly comprising an auxiliary drive motor may be operated to rotate the cutter drum independently of the primary drum drive assembly.