Windows NT 4.0 May Not Recognize SCSI Devices Using Nonzero LUNs

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Article ID: 162471 - View products that this article applies to.
This article was previously published under Q162471
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IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.
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Windows NT 4.0 may not recognize a SCSI device that uses a logical unit number (LUN) other than zero and that comes online after the system is started.

NOTE: Hardware RAID devices commonly use nonzero LUNs.


This problem is caused because the Scsiport.sys driver does not scan for non-zero LUNs if LUN 0 has already been detected.

For example, you have a system with a single SCSI disk drive at ID=0, LUN=0, and start the computer. Next, you turn on an external drive that is configured for ID=0, LUN=1. The Windows NT 4.0 system will not see this new device. Even if you send an IOCTL_SCSI_RESCAN_BUS call by running Disk Manager, the new device will not be found because of the problem with the Scsiport.sys driver.


Rescanning of nonzero LUNs requires two things:
  • Install WinNT 4.0 Service Pack 3 (or later) version of Scsiport.sys.

  • a LunRescan value added to the Disk key.
After these two things are in place, IOCTL_SCSI_RESCAN_BUS should find nonzero LUNs. The value under the Disk key is used even if the system has no disk devices installed.

Note that ScsiPort.sys will only find consecutive LUNs with the LunRescan registry entry, starting from LUN 0 up to the maximum LUNs supported on the device. As soon as a LUN does not respond, ScsiPort.sys stops scanning. For example, on a system with LUN 0, LUN 1, and LUN 5 responding, only LUN 0 and LUN 1 will be recognized. This behavior is by design.

To add this value to the Disk key, perform the following steps:

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys And Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it.

  1. Start Registry Editor (Regedt32.exe), and go to the following subkey:
  2. Add the following value to the Disk key:
    Value name: LunRescan
    Data type: REG_MULTI_SZ
The Data for this value can be left NULL and the IOCTL rescan will apply to all devices. To specify a specific device, the string in the value should match the device(s) Inquiry string. Only the matched string should be rescanned when the IOCTL is issued, but the SCSIPORT SP3 implementation does not limit rescan to only the specified device. However, this might change in a future release.

After this key is added, a reboot is NOT required. The IOCTL_SCSI_RESCAN_BUS should now search the nonzero LUNs.

Microsoft has confirmed this to be a problem in Windows NT version 4.0. This problem was corrected in the latest Microsoft Windows NT 4.0 U.S. Service Pack. For information on obtaining the service pack, query on the following word in the Microsoft Knowledge Base (without the spaces):


Article ID: 162471 - Last Review: October 26, 2013 - Revision: 2.0
Applies to
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 4.0 Standard Edition
kbnosurvey kbarchive kbbug kbsetup KB162471

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