FAT Disks

Hard drive data recovery in the event of a failure is extremely important because since about 1980 the majority of disks on a PC (ranging from floppy disks to hard disks and flash memory sticks / camera chips) have been based on a FAT based structure. FAT stands for File Allocation Table which is a series of pointers to where each cluster or section of the file is stored.

There are three variations, named after the number of bits in the pointer, FAT12, FAT16 and FAT32.  They have evolved to allow for larger disks with an ever larger number of files and total capacity.

FAT12 has 12 bit pointers giving a total 4095 possible clusters.  Even on a 1.4MB floppy with 2880 sectors this is a very viable system.  However, a limit of 4000 files on a modern hard disk would be laughed at, so currently most modern systems use FAT32 systems.  The FAT16 system ran out of capacity at about 2GB capacity.

Although FAT32 is still very common, NT, and XP have always had the option of a completely different file system called NTFS. The FAT system is fairly simple, but does not have the fault tolerance found in NTFS.  Thus a single sector failure can result in the disk being impossible to access from a PC.

A FAT disk consists of the following elements

  • A boot and partition sector (not on floppy disks)
  • A partition description sector
  • A FAT map or table
  • Directories and subdirectories
  • Data areas, ie files

Boot Sector

This is the very first sector on a hard disk and important details it gives includes the number size, and type of each partition of the disk. A PC hard disk can have up to 4 partitions that each act as if they were a separate disk drive. If this sector is missing or corrupted, the operating system will have no knowledge of what is on the disk, and will not be able to read the disk contents.  CnW can overcome this limitation.

Partition description sector

This can have several names such as Boot sector, or BIOS Parameter Block (BPB).  It stores very important information about the configuration of the partition including the number of FAT blocks, size of a disk cluster, total capacity of the disk and information to work out where the root directory is stored.  For FAT12 and FAT16 it also gives the size of the root directory, but on FAT32 systems the root directory is stored as a file and can therefore be any length. Again, a missing or corrupted sector makes it impossible for operating systems to restore data, thus requiring data recovery services.

FAT map

The FAT map defines which clusters of the disk are used, and the sequence of clusters.  Thus the directory entry will have the number of the first cluster, then the FAT mao is used to find the next cluster which could be stored anywhere on the disk, although whenever possible, they are are normally sequential.  If the FAT map is lost the disk becomes unreadable by the operating system, though CnW can still recover your files with a high level of accuracy.

Directory and subdirectories

Data areas

After all the above sections, one eventually gets to the reason for a mass storage device, ie files and data. Where ever possible, files are stored sequentially and with short files this is normally possible. It is far quicker to read 100s of sequential sectors rather than a few sectors, and then jump to a new location to read other sectors.

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