Knowledge Base

Cannot Boot to MS-DOS On a Windows NT Mirrored FAT Partition

Article ID: 145941

Article Last Modified on 11/1/2006

Microsoft Windows NT Server 3.5
Microsoft Windows NT Server 3.51
Microsoft Windows NT Server 4.0 Standard Edition

This article was previously published under Q145941

SYMPTOMS

Windows NT allows you to dual boot between Windows NT and another operating system, such as Windows 95 or MS-DOS. However, if you establish a mirror on the system partition that contains the two operating system files, MS-DOS stops responding (hangs) after it displays "Starting MS-DOS..." and the following appears when you start Windows 95:

The following file is missing or corrupt: Command.com
Type the name of the command interpreter.

CAUSE

MS-DOS looks for the System ID Byte in the partition table to determine what type of FAT file system to use. The system ID byte has three valid values that MS-DOS can determine:

Value     File System
-----     -----------
0x01      MS-DOS 12 Bit FAT
0x04      MS-DOS 16 Bit FAT
0x06      MS-DOS 16 Bit FAT (Greater than 32 MB in size)

When you set up fault tolerance in Windows NT on a FAT system partition, the high order bit of the system ID byte changes to 0x86, which MS-DOS does not recognize.

RESOLUTION

To work around this problem:

Use Disk Administrator to break the mirror and shut down Windows NT.
Reboot the computer and start MS-DOS or Windows 95.

NOTE: You may install Windows NT to another partition and establish the mirror.

Additional query words: prodnt

Keywords: KB145941