Repartitioning Corrupts Partition Table

• Microsoft Windows NT operating system, version 3.1
• Microsoft Windows NT Advanced Server, version 3.1

Disk Administrator (and the FDisk engine it shares with Setup) writes partition table information to the disk based on the disk's Cylinder-Head- Sector geometry. For SCSI disks, this geometry is invented by the driver, rather than being an inherent property of the disk. Windows NT follows a relatively standard method of using a geometry that produces 1-Megabyte tracks. However, other systems (or SCSI disks run with BIOS) can use different geometries.

When a disk with multiple partitions is taken from one system to another that uses a different geometry, repartitioning it with Disk Administrator can cause the partition table to become corrupt. This problem occurs because Disk Administrator aligns all partitions on track boundaries based on the current disk geometry. If the partition table entry for an existing partition is rewritten with a new starting offset, the data on that partition becomes inaccessible.

You can avoid or correct this situation by editing the partition table to set the partition starting sector and length back to its original value. You can use the BIOS Parameter Block (BPB) in sector one of the physical volume to determine the geometry, or you can use sector zero of the partition, which also contains a BIOS Parameter Block, to find the starting sector offset.