Magnetic disk device

Provided is a magnetic disk device capable of suppressing the time required for rewriting in units of tracks. In a first track of a magnetic disk, a plurality of first sectors and a second sector are arranged in the order of the plurality of first sectors and the second sector from a first position, each of the plurality of first sectors stores a data segment, and the second sector stores a parity for a first error correction. The controller executes a first operation in which first data segments are sequentially read from each of the plurality of first sectors, and groups of the read first data segments are stored in the buffer memory. The controller acquires a first parity from the group of the read first data segments. The controller starts a second operation that writes each of the first data segments in the group of first data segments stored in the buffer memory to a first sector of a read source among the plurality of first sectors and writes a first parity to a second sector before the head reaches a