A method of converting a hybrid hard disk drive (HDD) to a normal HDD when a system is powered on depending on whether the total number of defective blocks in a non-volatile cache (NVC) exceeds a predetermined threshold. The method of converting a hard disk drive (HDD) from a hybrid HDD to a normal HDD where the HDD has a normal hard disk and a non-volatile cache includes the steps of determining whether a mode conversion flag is enabled during a power-on period. When the mode conversion flag is enabled, operating the HDD as a normal HDD. When the mode conversion flag is disabled, determining whether an operating mode of the HDD is a normal mode or a hybrid mode. When the operating mode of the HDD is in the normal mode, the HDD operates as a normal HDD. A determination is made when the HDD is in the hybrid mode as to whether the total number of defective blocks in the non-volatile cache is greater than a predetermined threshold. The HDD is operated as a hybrid HDD when the total number of defective blocks is not greater than the threshold. The mode conversion flag is enabled and the HDD is operated as a hybrid HDD when the total number of defective blocks is greater than the threshold.