The present invention provides a data recording and reproducing apparatus capable of correctly performing positioning to the position where desired data is recorded, and a method for recording and reproducing data. The flexible coupling includes thin portions defining a portion of a heat transfer path from the drive shaft to the drive unit. Thus a GaN-based semiconductor device including a thin GaN film-joined substrate including a substrate different in type and a thin film of GaN joined firmly on the substrate different in type, and at least one GaN-based semiconductor layer deposited on the thin film of GaN, can be fabricated at low cost.