Fault Tolerant Boot Disk

Definition of fault tolerant boot disk in The Network Encyclopedia.

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What is Fault Tolerant Boot Disk?

A boot disk for a Microsoft Windows NT server whose system partition or boot partition is mirrored. The fault tolerant boot disk can be used to boot the system in the event that one member of the mirror set fails. If you plan to use disk mirroring in your system, you need to create a fault tolerant boot disk. Disk mirroring is the only form of fault tolerance that Windows NT and Windows 2000 support for the critical system and boot partitions.

How It Works

A fault tolerant boot disk should be formatted using a computer running Windows NT. This disk consists of a floppy disk containing the following files in its root directory:

  • Ntldr
  • Ntdetect.com
  • Ntbootdd.sys (for SCSI disks with BIOS disabled)
  • Bootsect.dos (on dual-boot systems only)
  • Boot.ini

In addition, the boot.ini file must be modified so that its ARC paths point to the mirrored copy of the system partition.


Test your fault tolerant boot disk by using it to boot your system from the shadow drive and making sure you can log on. Be sure to disable the primary drive of the mirror pair to perform the test. Also, if you change the system’s disk configuration using Disk Administrator, be sure to update the boot.ini file on the floppy.

Make sure that the primary and shadow drives of your mirrored pair are identical in make, model, and firmware revisions so that you can recover from a broken mirror set. However, this is not necessary if you have a valid fault tolerant boot disk.