Read Errors After Primary Member of Mirror Set Fails

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If the system experiences a failure on a mirror set, Windows NT will break the mirror and orphan the faulty drive. There may be a situation where bad data is passed back to applications after breaking the mirror. This can cause a variety of problems, such as application problems and read media errors on a healthy disk.

In addition to symptoms

  • A server with hot pluggable disk system (for example: Compaq, HP)
  • Mirror set created in the disk system
When you unplug the mirror disk while online from the mirror set (for example, hardware failure), a warning message is received that the mirror set is broke. The server and the original disk are functioning correctly. After you shut down the server, replace the mirror disk and then restart the system, you may encounter file corruption with the original disk. Disk Administrator may report a healthy status for the mirror set.


Although one of the mirrored disks was orphaned because of hardware failure, users continue to use the system without replacing the failed disk and re-creating the mirror set. During this time, disk I/O is suppose to be performed only from the partition of the healthy disk.

If an application requests data from the disk, and a read I/O error is encountered on the healthy disk, Ftdisk.sys erroneously retrieves data from the orphaned disk, which results in unreliable data from the orphaned disk being returned to the application as reliable data.


To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
152734 How to Obtain the Latest Windows NT 4.0 Service Pack


To work around the problem, before you shut down the server and replace the disk, you should manually break the mirror set in Disk Administrator.

For additional information, please see the following article in the Microsoft Knowledge Base:

120227 Steps to Recover a Failed Mirrored System/Boot Partition


Microsoft has confirmed that this is a problem in Windows NT 4.0 and Windows NT Server 4.0, Terminal Server Edition. This problem was first corrected in Windows NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition Service Pack 4.


Article ID: 194351 - Last Review: October 10, 2013 - Revision: 2.1
  • Microsoft Windows NT Server 4.0, Terminal Server Edition
  • Microsoft Windows NT Server 4.0 Standard Edition
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