To open Computer Management, click Start, and then click Control Panel. Click Performance and Maintenance, click Administrative Tools, and then double-click Computer Management.
You must be logged on as an administrator or a member of the Administrators group in order to complete this procedure. If your computer is connected to a network, network policy settings may also prevent you from completing this procedure.
To replace a mirror in the mirrored volume, you must have a dynamic disk with unallocated space that is at least as large as the region to repair. If you don't have a dynamic disk with enough unallocated space, the Add Mirror command is unavailable. (To verify you have enough space, right-click the disk, click Properties, and then check the size in Unallocated space. This size may be slightly smaller than shown in the graphical and list views.)
If you are replacing one of the disks in a mirrored system volume, be sure to use an identical disk to the one you are replacing. Otherwise, startup problems might occur if the disks use different geometries or if the system volumes are at different offsets on the disks. Always test your configuration by attempting to start from each volume to ensure that you can actually start if one of the disks fails. For more information about mirroring the system volume, see "Disk Management" at the Microsoft Resource Kits Web site.