If you are installing Windows NT on a mirrored partition, you must disable mirroring before deployment, and then reestablish mirroring after installation is complete.
Mirrored partitions are a feature you may have implemented in an existing installation of Windows NT Server, in which a copy of a partition on one disk drive is maintained on another disk drive. If the primary disk fails, no data is lost, since all information written to the primary disk was also written to the secondary disk.