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How to fix Windows NT 4.0's 7.8 GB System Partition limitation

Created: Jan 7, 2003
Updated: Mar 7, 2005

:: Introduction
:: Technical
:: Installation
:: Remarks


If you replace a harddisk from a NT 4.0 machine using Symantec Ghost or PowerQuest Drive Image, most of the time the new harddisk is a lot bigger than the old one. Normally with NT 4.0 you have to "clone" the NTFS system partition within the 7.8 GB limit. But not anymore...

This page will show you how you can use Windows 2000 or XP's bootsector, ntldr and ntdetect.com to fix NT 4.0's 7.8 GB system partition limitation.

To pull this off you'll need a tool, mkbt.exe created by Bart Lagerweij. For more information on mkbt check the mkbt page.

Mkbt.exe is used to extract or install FAT and NTFS partition boot sector(s).


Although Windows NT 4.0 can in theory support partitions of up to 16 exabytes in size using the NTFS file system, the maximum size of the system partition is limited to 7.8 gigabytes (GB).

What is the system partition?
The system partition is defined as the partition containing the files needed for the initial system startup. For Windows NT, the files are Ntdetect.com, NTLDR, Boot.ini, and sometimes Ntbootdd.sys

Why the 7.8 GB limit?
During the bootstrap process, Windows NT 4.0 uses INT13 BIOS functions to access the drive. The INT13 functions have a maximum of 256 heads, 1024 cylinders, and 63 sectors. This is approximately 7.8 GB (512 bytes per sector).

NOTE: Partitions other than the system partition are not affected by the these limitations.

When trying to boot with ntldr located passed the 7.8GB limit you will get this error message:

Os Loader V4.01
.Disk I/O error: Status = 00000001

Windows NT could not start because the following file is missing or corrupt:
Please re-install a copy of the above file.

More information about this:

Windows NT 4.0 Supports Maximum of 7.8-GB System Partition
Microsoft Knowledge Base Article - 224526

Windows NT Boot Process and Hard Disk Constraints
Microsoft Knowledge Base Article - 114841


The steps to fix NT4.0 are:
  1. Download mkbt.exe from the mkbt page and put in on a floppy disk.

  2. On a Windows 2000 or XP PC copy ntldr and ntdetect.com. They are normally located in the c:\ folder and hidden. Copy them to a floppy disk. Use mkbt.exe to extract the NTFS boot sector(s). A bootable NTFS partition uses the first 16 sectors to store the bootstrap loader.

    Use this command to extract the NTFS bootstrap loader and save it to floppy disk:
    mkbt -x -c c: a:\bootsect.bin

    The -c option is for "copy" mode.
    The -x option is for "expert" mode, in standard mode mkbt only accepts floppy drives "a:" and "b:"

  3. Goto to the "limited" NT 4.0 system, boot it and login as administrator.

    Rename the existing ntldr into ntldr.nt4
    Rename the existing ntdetect.com into ntdetect.nt4
    Copy the ntldr and ntdetect.com from the floppy disk on the NT 4.0 system partition, normally "c:\".

    Use this command to make a backup of the current boot sector:
    mkbt -x -c c: a:\bootsect.nt4

    Use this command to install the NTFS bootstrap loader from floppy disk:
    mkbt -x a:\bootsect.bin c:

  4. If your NT 4.0 machine uses a fixed NT boot disk driver ntbootdd.sys (will have "scsi" entries in your boot.ini) you must also update that to a Windows 2000 or XP version.
Done. You can now reboot the system and see if it boot's up OK. With the Windows 2000 or XP bootstrap loader your Windows NT 4.0 machine can have a system partition of 2TB. This is the maximum partition value.


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