JSI Tip 1990. Why are identical drives on identical controllers sized differently?.

When Windows NT boots, the mini-port drivers, SCSI disk, and class drivers load. EIDE drives behave the same way as the ATAPI driver makes EIDE devices look like SCSI to the system.

These components query the SCSI or computer BIOS for a drive translation. Most modern SCSI devices return a generic 255 heads and 63 sectors per track, leaving the number of cylinders as the remaining size variable. Per tip 1295, this allows for a 4GB FAT or 7.8GB NTFS System partition. With this translation scheme, the boot sector for the first partition is normally located on physical sector 63.

When you have multiple controllers, the BIOS translation scheme is only used on the drives of the boot controller. Windows NT defaults the drives on the other controller(s) to 64 heads and 32 sectors per track. This places the boot sector for these drives on sector 32, making the drives look 31 sectors larger than if the 255/63 schema had been used.