According to one embodiment, a magnetic disk device includes a disk, a head including a read head that reads data from the disk, a write head that writes data to the disk, and an assist element that generates energy to enhance write performance by the write head, and a controller that selects and performs a first recording mode and a second recording mode different from the first recording mode, and selects and performs one of the first recording mode and the second recording mode according to an assist effect of the assist element.

According to one embodiment, a magnetic disk device includes a disk, a head including a read head that reads data from the disk, a write head that writes data to the disk, and an assist element that generates energy to enhance write performance by the write head, and a controller that selects and performs a first recording mode and a second recording mode different from the first recording mode, and selects and performs one of the first recording mode and the second recording mode according to an assist effect of the assist element.