A disk drive capable of data transfer with a magnetic disk at, say, 192 tracks per inch (tpi) is made compatible with a 96 tpi disk drive. The 192 tpi disk drive comprises a magnetic head assembly having both a read/write head and a pair of tunnel erase heads. During writing at 192 tpi the tunnel erase heads are held out of operation, making it possible for the read/write head to form data tracks at the high density required. The tunnel erase heads are automatically switched on and held in operation during writing at 96 tpi, for erasing the edge portions of each data track being created by the read/write head. Thus, even if the disk on which data is to be written at 96 tpi bears old data tracks that have been prerecorded by a 96 tpi disk drive, such old data tracks will be completely erased by the pair of tunnel erase heads as the 192 tpi disk drive rewrites thereon at 96 tpi. So rewritten, the disk can be read by either the 192 or 96 pti disk drive.