The FAT system stored all control information on 1st track (0) to optimize 
  access. But if 1st track damaged, files lost.

Duplicate FAT
  The Microsoft OS keeps a second copy of the FAT. 

  On earliest versions, FAT 12 (floppy), no duplication.

  On FAT 16, both copies next to each other.
    Also possible for tables to hold dis-similar info.
  
  On newer FAT 32, copies may be separated to protect against corruption.
    Auto-Synchronization of fat provided but can be disabled.