The first sector of a partition is call a Volume Boot Record, or VBR. The purpose of the volume boot record is to describe a FAT file system. It does so with the following data:
The DOS version string is an 8 byte ASCII text string that identifies the manufacturer and version of the operating system used to create the file system.
The bytes per sector value represents the number of bytes that are in each sector of the media that the file system was created on. This is typically set to 512.
The sectors per cluster value represents the number of physical sectors that are in each cluster. Clusters are the basic allocation unit of the FAT file system. See File Allocation Tables, for more information on clusters.
In the FAT12 and FAT16 file systems, a root directory is a fixed size. This value represents the number of 32 byte entries the root directory can hold. FAT32 systems support a variable size root directory, and therefore this field is meaningless in that file system.
Some DOS version use the media descriptor byte to determine the characteristics of the disk drive in which the partition resides. For example hard disk drives will have a media descriptor byte value of F8h and a 3 1/2" floppy disk may have a value of F0h.
The volume serial number is a 32-bit randomly generated number used to uniquely identify a file system.
The volume label string is an 11 byte ASCII text string that identifies the
file system to DOS. This value is input by the user upon execution of the
format
command.
The file system ID string is an 8 byte ASCII text string that describes the file system that resides on that partition. One must be aware, however, that operating systems do not use this string to determine the file system of the drive and is essentially useless.