Microsoft KB Archive/139843

= Recovering from Stop 21A Caused by Disk Mirror Sets =

Article ID: 139843

Article Last Modified on 11/1/2006

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APPLIES TO


 * Microsoft Windows NT Advanced Server 3.1
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q139843



SYMPTOMS
If the Hkey_Local_Machine\System\Disk key gets corrupted or the Microsoft Windows NT disk signature on the system drive is removed or changed (and the system drive is part of a Windows NT software level mirror set) you may receive the following error message and stop message upon restart:

Autochk program not found. Skipping autocheck.

Immediately following the error message above, the next error message will appear:

STOP: c000021a {Fatal System Error}

The Session Manager Initialization system process terminated unexpectedly with a status of 0xc0000013 (0x00000000 0x00000000).

The system has shutdown.



CAUSE
This is caused by the File system indicator byte in the master boot record (MBR) of the boot drive (that is set to Fault tolerant) but the Windows NT registry does not contain FT Information about the drive or partition that is mirrored. This can also be caused by a corrupt Ftdisk.sys device driver.



RESOLUTION
If you have the Windows NT 4.0 Resource Kit, you can use the Disksave.exe utility to remove the fault-tolerant bit in the MBR.

To remove the fault-tolerant bit in the MBR, perform the following steps:


 * 1) Restart your computer from an MS-DOS version 6.XX Disk.
 * 2) Run the Disksave.exe utility, and select the F6 option.

This can reset the high order bit of the system ID Byte of the active partition on disk zero (boot drive only). This does not work if Windows NT is installed on a logical drive in an extended partition.
 * 1) Quit Disksave and restart the computer to Windows NT.
 * 2) When you enter Windows NT, go to Disk Administrator and break the mirror, restart and re-establish the mirror.

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To fix this problem, you need a third-party disk editor, such as Norton Diskedit.

To change this, do the following:

NOTE: Norton diskedit is used in the following example.


 * 1) Format a floppy disk with MS-DOS 5.0 or greater using the /s switch to make the disk bootable.
 * 2) Find a copy of Norton Diskedit and place Diskedit.exe on the floppy disk.

NOTE: Make sure the floppy disk is scanned with the latest virus software and write protect the disk after formatting. Booting with a floppy disk is one way that viruses are transferred to the boot drive.

 Place the floppy disk in the drive and start the computer. Run Diskedit from the floppy disk.  On the Object menu, select DRIVE.. Select TYPE - Physical Disk and select Hardisk1.

NOTE: The BIOS must be enabled on the SCSI Host Bus adapter to see the Physical drive, click OK.

This dumps you out at Cyl 0 Side 0 Sector 1, this is the Master Boot Record of the disk. Go down to line 000001C0: and find the third byte, this is the File System Indicator.

NOTE: If you have a Compaq computer with an EISA partition, go down to line 000001D0.

It has one of two values:

86 = Fat mirror

87 = NTFS mirror

 Change this value to 06 or 07 depending of the type of file system on the drive. On the Edit menu, click Write Changes.</li> Quit Diskedit and restart the computer to Windows NT. When entering Windows NT, go to Disk administrator and break the mirror, restart and reestablish the mirror.

NOTE: If more than one mirror set was on the system, they can show up in disk administrator without a drive letter. Just assign one in disk administrator.</li></ol> </li></ol>

NOTE: The MS-DOS boot disk must be formatted on an NTFS system.

Additional query words: prodnt fault tolerence ft

Keywords: kbenv kbhowto KB139843

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