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Windows 2000 assigns duplicate drive letters to a single USB floppy drive

A Microsoft Windows 2000-based computer may incorrectly assign duplicate drive letters to a single universal serial bus (USB) floppy drive when you attach the USB floppy drive to the computer and then restart the computer. Windows Explorer may display duplicate drive letter icons for a single USB floppy drive, and one of the drive letters may not be valid for the USB floppy drive.

This problem occurs because of a synchronization issue between the Windows 2000 kernel and the MountMgr.sys driver. When you attach a USB floppy drive to a Windows 2000-based computer, Windows 2000 uses the following procedure to assign drive letters to the USB floppy device:
  1. When the computer starts, Windows 2000 enumerates the USB floppy disk drive.
  2. During system initialization, the Windows 2000 kernel calls the IoAssignDriveLetters function to grant drive letters to all present storage devices, such as hard disks, CD-ROM drives, and floppy drives.
  3. To assign the drive letters to the devices, the IoAssignDriveLetters function calls the HalpNextDriveLetter function to assign a drive letter to the USB floppy drive.
  4. The HalpNextDriveLetter function tries to call the HalpSetMountLetter function for the USB floppy drive and issues a IOCTL_MOUNTMGR_CREATE_POINT command to assign a drive letter.
  5. The MountMgr.sys driver uses the IOCTL_MOUNTMGR_CREATE_POINT command to create the mount point for the USB floppy device, but the driver cannot assign a drive letter because, when the MountMgr.sys driver tries to call the IoGetDeviceObjectPointer function for the USB floppy drive, the procedure fails. The failure occurs because the USB floppy drive still has the DO_PENDING_START flag set. The DO_PENDING_START flag means that the USB floppy device has not yet responded.
  6. Because the HalpNextDriveLetter function cannot assign a drive letter with the IOCTL_MOUNTMGR_CREATE_POINT command, the HalpNextDriveLetter function calls the IoCreateSymbolicLink function and creates a symbolic link for the drive letter that has no corresponding registry entry. This letter is the first drive letter that is assigned to the USB floppy drive.
  7. After the symbolic link has been created, the MountMgrMountedDeviceArrival function is called to notify the MountMgr.sys driver that the USB floppy drive has arrived.
  8. The MountMgr.sys driver is unaware of the symbolic link that is created by the HalpNextDriveLetter function, and therefore the MountMgr.sys driver assigns the next available drive letter. This letter is the second drive letter that is assigned to the USB floppy drive.
After you apply this hotfix, the MountMgrMountedDeviceArrival function checks for an existing USB floppy drive that has the same symbolic link as the one that is arriving. If a symbolic link exists for the USB floppy drive, but no corresponding registry entry exists for the drive letter, the previously assigned symbolic link is removed, and the MountMgrMountedDeviceArrival function assigns the drive letter correctly.

Hotfix information

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, submit a request to Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.


No prerequisites are required.

Restart requirement

You must restart your computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any other hotfixes.

File information

The English version of this hotfix has the file attributes (or later) that are listed in the following table. The dates and times for these files are listed in coordinated universal time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.
   Date         Time   Version            Size    File name   --------------------------------------------------------------   10-Feb-2004  19:47  5.0.2195.6897      30,160  Mountmgr.sys
To work around this problem, follow these steps:
  1. Make a full system backup of the computer and system state.
  2. Log on as administrator.
  3. Click Start, click Run, type regedt32.exe, and then click OK.
  4. Locate the following registry key:
  5. Click MountedDevices, and then click Permissions on the Security menu.
  6. Verify that the Administrators group has Full Control permissions.

    Note If the permissions are modified, make sure that they are returned to their original settings.
  7. Quit Regedt32.exe.
  8. Click Start, click Run, type regedit.exe, and then click OK.
  9. Locate the following registry key:
  10. Find the drive letters that overlap. For example, if "B:" overlaps, find "\DosDevices\B:", right-click "\DosDevices\B", and then click Delete.

    Note You must use Regedit.exe instead of Regedt32.exe to delete this registry key.
  11. Quit Regedit.exe.
  12. Click Start, click Run, type regedt32.exe, and then click OK.
  13. Locate the following registry key:
  14. Change the permissions back to the previous settings for the administrators group.
  15. Install the hotfix that is listed earlier.
  16. Restart the computer.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section of this article.
For additional information, click the following article numbers to view the articles in the Microsoft Knowledge Base:
816915 New file naming schema for Microsoft Windows software update packages
824684 Description of the standard terminology that is used to describe Microsoft software updates

Article ID: 836662 - Last Review: 10/26/2006 20:48:18 - Revision: 3.5

  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Professional Edition
  • Microsoft Windows 2000 Advanced Server
  • kbautohotfix kbhotfixserver kbqfe kbbug kbfix kbqfe kbwin2000presp5fix KB836662