Alternate Methods of Booting from a Failed Mirror (129042)

This article was previously published under Q129042

If the primary or boot drive is mirrored and fails, the following alternate method can be used to boot from the mirrored drive if there is no fault tolerant boot floppy available. This procedure applies only if the boot partition is mirrored. You need MS-DOS FDISK.EXE or Norton Utilities DISKEDIT.EXE. This method works only fox x86-based platforms.

1. Remove the power cable from the failed primary drive.
2. Boot from drive A.
3. To use FDISK:
   
   
   1. Start FDISK. Make sure you have Fixed Disk Drive 1 selected.
   2. Select option 2, Set Active Partition.
   3. Press 1 when you are prompted to enter the number of the partition you want to make active. Enter 1.
   4. When the Partition 1 Made Active message appears, press Esc. FDISK reboots the system.
   
   
   To use DISKEDIT:
   
   
   1. From the Object menu, choose Drive.
   2. Select physical disks, hard disk 1.

      This shows the MBR on the disk.

   3. Go to offset 446.
   4. This byte will be set to 00, change it to 80.
   5. From the Edit menu select Write changes and then select Write.
   6. Quit DISKEDIT.

You should now be able to boot from the hard disk.

NOTE: The specified offset for the boot indicator assumes that the first physical partition table entry contains the system partition. This may not always be the case. Some computers, such as Compaq, may have an EISA configuration or other partition not used by the operating system as the first physical partition. The system partition has the files NTLDR, NTDETECT.COM and BOOT.INI. This is the partition that should be marked active. The correct offset can be calculated by adding 16 for the next partition table entry or using the table below.

Boot Indicator for Partition 1 = 446 - Hex 01BE
Boot Indicator for Partition 2 = 462 - Hex 01CE
Boot Indicator for Partition 3 = 478 - Hex 01DE
Boot Indicator for Partition 4 = 494 - Hex 01EE