Recovering A Mirror Set

Recovery for a mirror set that is not a system partition is easy—as long as both drives did not fail, that is. When the master drive fails, the slave drive automatically is given the drive letter of the master drive. If a slave drive fails, there is no change for drive letters, but the mirror set is no longer functioning. The basic procedure to recover a mirror set is outlined in the following steps.

1.  Launch Disk Administrator, and choose Break Mirror from the Fault Tolerance menu.

2.  When prompted to confirm this decision, choose Yes.

3.  Make an immediate backup of the data on the remaining drive.

4.  Shut down the computer.

5.  Replace the failed drive. If the master failed, swap the slave disk for the master disk. If required, change the SCSI ID on the slave disk to match the master disk. If the slave disk failed, replace it with another disk of the same or larger capacity, and set the SCSI ID to the same ID that the slave drive previously used. If using a SCSI controller, low-level format the drive.

6.  Restart the computer.

7.  Re-create the mirror set using the instructions outlined previously in the section entitled Creating A Mirror Set.

If the mirror set is also your system partition, it takes more work to rebuild the mirror set. This process is discussed in the following section.

Recovering A Mirror Set For A System Partition

Now that the basics are out of the way in understanding ARC pathnames, you can put this knowledge to good use. The first step in recovering from a failed mirror set of a system partition is to modify the ARC pathnames for your startup disk. You have to change the ARC pathname to point to the slave (shadow) partition where your copy of Windows NT resides. This requires that:

•  Both disk controllers are the same model and have the same selected translation method.

•  Both disk drives are the same model (or at least the same geometry—number of cylinders, number of heads, number of sectors per track, and so forth).

•  The system partition is the same as the boot partition and has failed; the boot partition is different from the system partition, and the system partition has failed; or the boot partition is different from the system partition, and the boot partition has failed.

The last requirement in the bulleted list is where the fun really begins. In each scenario, you can try a shortcut to starting Windows NT by using the startup disk. This would only be required if you have to restart your computer before you are ready to replace the failed disk. To do so, you might have to modify the ARC pathname in the BOOT.INI on the startup disk to point to the Windows NT location. Modifying the ARC names, of course, depends on which partition fails.

If the boot partition fails, you can try and rebuild it. You can use an MS-DOS disk to boot the computer, reformat the partition, and rebuild the partition using the repair procedure provided by the Windows NT Setup program. This requires the three original installation disks or a set made by running WINNT /ox (under MS-DOS) or WINNT32 /ox (under Windows NT) from the I386 directory on the installation CD-ROM. Choose only to inspect the startup environment, and inspect the boot sector. This will replace the Windows NT boot sector, NTLDR, NTDETECT.COM, NTBOOTDD.SYS (if required), and BOOT.INI.

If the system partition fails, you can try using the Windows NT startup disk you created to boot from the slave disk. Simply change the ARC name in the BOOT.INI to point to the location of Windows NT on the slave drive. In most cases, this will get you up and running immediately.

If the system partition is the same as the boot partition and it fails, the master partition (the original part of the mirror set) has failed and you are running on the slave partition. This means you should replace the disk as soon as possible. When you are ready to replace the disk, you should follow the steps presented in this section:

1.  Launch Disk Administrator, and choose Break Mirror from the Fault Tolerance menu.

2.  When prompted to confirm this decision, choose Yes.

3.  Make an immediate backup of the data on the remaining drive.

4.  Use DISKPROBE.EXE (on the Windows NT Server Resource Kit) to set the boot indicator flag.

5.  Shut down the computer.

6.  Swap the slave disk for the master disk. If required, change the SCSI ID on the slave disk to match the master disk.

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Note:  On a computer using EIDE drives, you should change the master/slave jumper instead of the SCSI ID.

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7.  Use your MS-DOS startup disk to reboot the computer.

8.  Use DISKSAVE.EXE to restore your boot sector. If you have not changed the partition table since your last save of the boot sector, you do not have to use DISKSAVE.EXE to restore the master boot record. If you have changed the partition table and saved a copy of the master boot record with DISKSAVE.EXE, you can use that copy to replace the master boot record. If you do not have a saved copy of the master boot record, you can use FDISK /mbr to rebuild it.

9.  Remove the MS-DOS startup disk, and restart the computer.

If the preceding steps do not work, you are in trouble. What you can do at this point varies depending on the severity of the error. In a worst-case scenario, you will have to place the slave disk back in its original position with the original SCSI ID. Then, install a new disk of the same model as the master disk, and reinstall NT to the new master disk. You can see if NT can recover any information on the slave disk. If so, you can back it up and then restore it on the new master disk. This will replace the new installation of Windows NT with the old version of Windows NT. Then, you can delete the partition on the slave drive and re-create the mirror set. Recovering a stripe set with parity is less traumatic, as you will see in the next section.