Windows NT 4.0 has a 7.8GB System partition limit.
This limit has been removed in later versions of Windows NT-based operatings systems (Windows 2000 (NT 5.0), Windows XP (NT 5.1), and Windows .NET (NT 5.2)).
You can use later versions of the boot files and Bart Lagerweij's mkbt freeware to overcome Windows NT 4.0's 7.8GB System partition limit:
01. Download the mkbt20.zip file and extract mkbt.exe.
02. On a computer with NT 5.x boot files, copy NTLDR and NTDetect.com from the root of the System partition to a diskette. If Windows NT 4.0 uses Ntbootdd.sys, you will also need to copy the SCSI driver to the floppy as Ntbootdd.sys.
03. Extract the Windows NT 5.x bootstrap loader to a floppy, using:
mkbt -x -c c: a:\bootsect.bin.
NOTE: A bootable NTFS 5.x partition uses the 1st 16 sectors to store the bootstrap loader.
04. Log on to the Windows NT 4.0 computer with Administrative authority.
05. Open a CMD prompt.
06. Type Rename c:\ntldr c:\ntldr.old and press Enter.
07. Type Rename c:\ntdetect.com c:\ntdetect.old and press Enter.
08. If it exists, rename c:\ntbootdd.sys to ntbootdd.old.
09. Copy ntldr and NTDetect.com from the floppy (step 02) to the root of the C: drive.
10. If you created a ntbootdd.sys in step 02, copy it from the floppy to the root of the C: drive.
11. Run mkbt -x a:\bootsect.bin c: to install the Windows NT 5.x NTFS bootstrap loader to the C: (System) partition of the Windows NT 4.0 computer.
12. Windows NT 4.0 should now boot.