Had to do this several times, I'd suggest making the old mirrored secondary drive (the one that is still good) the new primary drive. Then mirror off from that drive. To do that, just do the following...
Get out your Emergency Repair Disk. If youdon't have one, run 'rdisk' and create one OR update your old one. You will also need your setup floppy disks, NT CD, and special drivers (i.e. SCSI). I also presume you have a good backup.....
Break the mirror, take out the bad drive, leaving the good one in, configure it on the same controller in the same position as the primary/bad drive was, i.e. primary master if ide.
Use your NT boot disks to get to the main setup screen, where it should ask you to; begin installation, begin custom installation, leave setup or repair a damaged system. Choose repair of course (press 'R').
You will get a prompt for the repair disk, insert it into floppy drive. It will read then ask for setup disk #1, give it the disk. After a little more reading it will ask you to; Inspect NT startup env., verify windows server system files, verify the boot files on c:, inspect the configuration registry files. You can go with the default selections, which is all the above.
After some more reading, itwill ask for Setup disk #3 for device drivers (and if you use SCSI, this will be the time to load the drivers), then once its finished loading the drivers, it will ask for the repair disk again.
Load the disk in, it will 'examine your drive c:' When it is done, take out all floppy disks and cd, then press 'ENTER' to restart as the prompt says.
You should now beable to boot up with your good disk as drive c:. You can then use your good copy to mirror off from.
Hope that helps and good luck.
