NTFS Large Disk Boot Problems

If your NT system partion or first partition on that disk is too large you may find it suddenly won't boot .

What's the cause of the problem

Windows NTs NTFS disk format allows large partitions. With a 512 byte cluster size, you can format a partition of 2 terabytes (TB). The maximum 64KB cluster size allows 256TB. But even though Windows NT can address this, the boot process starts with your BIOS (Basic Input/Output System), not Windows NT.
The BIOS locates the beginning of a partition by using three numbers: The Starting Side (or Head), the Starting Cylinder, and the Starting Sector. The end of a partition is identified by three similar numbers. Now, the Side value is 8 bits, and can range from 0 to 255 (256 numbers); the Cylinder is 10 bits, and can range from 0 to 1023 (1024 numbers); the Sector is 6 bits, and can range from 1 to 63 (63 numbers). (Note that zero is not a valid sector number.) This means the maximum address on the disk is Side 255, Cylinder 1023, Sector 63. The number of sectors is 256 X 1023 X 63, or almost 16.5 million sectors. Standard sectors are 512 bytes, so we have a size of 7.87GB.
Thats the point: The BIOS cannot access anything beyond the first 7.87GB of the hard disk. If any critical boot data, such as the files NTLDR, NTDETECT or BOOT.INI, get moved to a point more than 7.87GB from the start of the hard disk, your computer will not boot. Anything that moves one of the critical files may cause the problem; you may copy the file from another partition, or you may edit the file. But you will not be aware of any problem till the next time you boot.
You are vulnerable to this if the boot partition for Windows NT is larger than 7.87GB, or if this partition is not the first partition on the disk and the total of this partition and all of the partitions before it exceeds 7.87GB.

Fixing the Problem

Presumably you have discovered this problem as you have just tried rebooting. If you have a boot floppy with copies of NTLDR, NTDETECT.COM and BOOT.INI on it this will get the system up and running immediately. The BIOS accesses the floppy and finds the data it needs, then the floppy starts Windows NT. Of course, the next time you boot, the problem is back, but this gets you going.
To fix the problem at your convenience, or if you haven't previously created a boot disk. Start with your Windows NT Setup Floppies and CD, and proceed as if you were installing a second Windows NT installation. When asked which folder to use, specify a new folder, not the one that the existing Windows NT is in. When asked if you want to check the hard disks for errors, select yes. Once this check is done, you can abort the new installation and Windows NT will boot up successfully.
Another way of handling this is to move the critical files back closer to the beginning of the disk. This can be difficult because you really dont know where the files will be placed when you copy or move them. However, the odds are on your side if the partition lies mostly within the 7.87GB limit. Trial and error will do it.

Preventing the Problem

The simplest prevention is to always have the boot partition as the first partition on the hard disk, and ensure that it is less than 7.87GB. Make sure you have a NT boot floppy for your system.
For additional information please see the following references:-

ARTICLE-ID: Q114841
TITLE : Windows NT Boot Process and Hard Disk Constraints

ARTICLE-ID:Q127134
TITLE : Windows NT Setup: SCSI Boot Disk Size Limitations

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Last modified: 04 June 2005