FAT file system-Backup Boot Sector

Another feature on FAT32 volumes that is not present on FAT16/FAT12 is the BPB_BkBootSec field. FAT16/FAT12 volumes can be totally lost if the contents of sector 0 of the volume are overwritten or sector 0 goes bad and cannot be read. This is a “single point of failure” for FAT16 and FAT12 volumes. The BPB_BkBootSec field reduces the severity of this problem for FAT32 volumes, because starting at that sector number on the volume—6—there is a backup copy of the boot sector information including the volume’s BPB.

In the case where the sector 0 information has been accidentally overwritten, all a disk repair utility has to do is restore the boot sector(s) from the backup copy. In the case where sector 0 goes bad, this allows the volume to be mounted so that the user can access data before replacing the disk.

This second case—sector 0 goes bad—is the reason why no value other than 6 should ever be placed in the BPB_BkBootSec field. If sector 0 is unreadable, various operating systems are “hard wired” to check for backup boot sector(s) starting at sector 6 of the FAT32 volume. Note that starting at the BPB_BkBootSec sector is a complete boot record. The Microsoft FAT32 “boot sector” is actually three 512-byte sectors long. There is a copy of all three of these sectors starting at the BPB_BkBootSec sector. A copy of the FSInfo sector is also there, even though the BPB_FSInfo field in this backup boot sector is set to the same value as is stored in the sector 0 BPB.

NOTE: All 3 of these sectors have the 0xAA55 signature in sector offsets 510 and 511, just like the first boot sector does