Recover From a Broken Mirror Set
Have your NT Emergency Repair disk ready that points to the second (mirrored) partition as the primary partition instead of the failed primary.
To accomplish this have a floppy that can be booted to NT on which the BOOT.INI file has been modified to point to the mirrored partition. (Remember to set the file attributes to remove the “read only” so you can modify the BOOT.INI)
Boot machine then install a new drive where the primary used to be.
After you obtain an identical new drive, use the NT Disk Administrator to format and remirror (i.e. mirror the mirrored drive to the new primary) then reboot.