Restoring Boot Partition Boot Sector Using Uninstall (76899)

The information in this article applies to:
    Microsoft MS-DOS operating system 5.0
    Microsoft MS-DOS operating system 5.0a
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.2
    Microsoft MS-DOS operating system 6.21
    Microsoft MS-DOS operating system 6.22

This article was previously published under Q76899


The primary DOS partition on a hard disk has its own boot sector that contains information on the format of the partition and the boot strap code. If the Uninstall program does not complete successfully, this boot sector may not be restored, which can cause an error message (for example, "non-system disk error") to be displayed. The primary partition's boot sector can be restored from the Uninstall disk using Debug.


The primary partition boot sector describes the partition format and contains boot strap code. This sector is the first logical sector on the partition. The old primary partition's boot sector is stored in the PBOOT.DAT file on the Uninstall disk. DOS interrupt 26H (absolute disk write) can be used to write this information back to the first sector of the boot partition.

CAUTION: Use this procedure with caution and only as a last resort. In most cases, the SYS command is adequate.

To write the old primary partition boot sector back to the first sector of the boot partition, do the following:

    Place the Uninstall disk #1 into drive A.
    Start the Debug program by entering the following at the command prompt:

    debug a:\pboot.dat

    At the hyphen (the Debug prompt), enter the following commands:
    Debug        Command
    Prompt       to Enter           Comments
     -            w cs:100 2 0 1     Write pboot.dat (at cs:100) to C;
                                        execute the command at cs:ip.
     -            q                  Exit Debug
The boot sector on the primary DOS partition is now replaced by the boot sector that was there before the MS-DOS 5, 6, or 6.2 Upgrade was installed.

Modification Type: Major Last Reviewed: 5/12/2003
Keywords: KB76899