Mirrored drive replacement

Recently, one of my servers (NW 3.20) two 18 gb mirrored (not duplexed) SCSI drives crashed. I determined which drive had crashed, removed it, rebooted and everything seems to be OK. I have ordered a replacement drive. NOTE: The server boots fromthe SCSI drive, no an IDE drive. The problem is that I have never had to replace a mirrored drive. Some guidance would be appreciated. Do I format the drive? If so, do I use NEtware or DOS? Do I create a DOS partition on the new drive? Do I install DOS on the new drive?
Thank you,
Mike Peck