Create a boot disk by copying ntldr, NTdetect.com and ntloader and boot.ini from a pc that has the same configuration as the one with the broken mirror, if you can't do that but can copy these files to a floppy disk open boot.ini and change the arc paths to the same as what they were on your server, but instead of exactly the same make sure that the arc path point to the good drive.
Boot from floppy.
Once in the server open Disk Manager and break the mirror, shutdown the server and replace the drive.
Boot it back up and recreat the mirror.
Once this is done you can then either keep the arc path as what it is or change it back to point to the new drive that was just inserted.
Hope this helps.