A redundant recording disk device includes a first logical sub-disk and a
second logical sub-disk. Mirrored redundant data are recorded such that a
recording position on the first logical sub-disk and a recording position
on the second logical sub-disk are positions on a corresponding radius and
have a phase difference of 180 degrees around respective rotation axes of
the disks with respect to one data. Therefore, the practical use of the
disk device can be continued even when one recording surface is not usable
because of a fault, and the access speed of the disk can be increased when
there is no fault since the mirrored data which have been written with a
predetermined phase-offset into the different recording surfaces may be
read out in a parallel manner.