Magnetization reversal dynamics in single perpendicular Pt/Co/Pt high aspect ratio nanodisks have been studied by polar magneto-optical Kerr effect microscopy. The He+ ion process used to pattern these nanodisks resulted in a soft/hard nanodisk structure. The energy barrier field dependence was determined from the simple exponential variation of the nonswitching probability with time. The standard coherent reversal scenario fails to account for it whereas a two-dimensional-confined droplet model supports the experimental observations. From a ring shaped metastable domain wall in the magnetically soft outer ring, the reversal progresses via the deformation of the domain wall and the expansion of a reversed confined droplet through the nanodisk hard core. This magnetic reversal, governed by domain wall propagation alone, ensures a magnetization reversal reproducibility from nanodisk to nanodisk and the very narrow energy barrier distribution observed.