Altirium logo

DOS FAT Hard Drive Data Recovery

About MSDOS, Win95 & Win98 File Systems

FAT is the catch-all name for a set of file systems used in MS DOS and Windows that are based around a File Allocation Table. The allocation table maps out the file system space occupied by any file or directory, with each entry pointing to the next entry in the chain that comprises the file, and with the final entry being a terminator. Each entry relates to a cluster, which is the internal allocation unit within the file system. A cluster will be a number of hard disk drive sectors, since there are a finite number of allocation entries, the FAT is fixed in size, to enable larger files to be stored the cluster size must be large. Since a cluster can only be used by one file or directory this does give rise to the problem that a FAT file system with large clusters is not good for storing very large numbers of smaller files as the amount of space wasted is large.

There are usually 2 copies of the FAT, this is an attempt to give some protection against damage to the primary copy.

FAT file systems are compact and require comparatively few resources to maintain, so rather than die out they have thrived in the consumer electronics world. Camera cards and personal music players often using FAT or variants of it.

FAT12 was introduced with the first version of MSDOS, with each FAT comprising a sequence of 12 bit entries. With 12 bit FAT entries the maximum addressable cluster is 4,095 which with a small cluster size (1 sector per cluster) allows only for the file system to be up to 2MB. Since FAT12 was introduced when there was only floppy disk support this was not a problem, and FAT12 is still used with diskettes as it does not waste a lot of space.

FAT16 was introduced with the advent of hard disk drive support for the PC under MSDOS. 16 bit FAT entries allowed up to 65535 clusters to be addressed. Since cluster sizes under FAT16 can be up to 32KB, this allowed for file systems up to 2GB, which at the time was more than seemed ever likely to be needed.

FAT12 and FAT16 came with other restrictions. Originally FAT was a single level file system with no sub-directories, the root (and only) directory was a space of fixed size which could store a finite number of files. These files were limited in the characters that could be used, and to 8 bytes with a 3 byte extension, hence the term "8.3", for file names.

FAT32 was introduced to break away from some of the size restrictions and name limitations of FAT16. There is still a 4GB limit on the size of any single file, but up to 4,294,967,295 clusters can be addressed within the FAT so the file system can exceed 2GB, also the directory structure has been enhanced to allow longer file names so the 8.3 limit has been broken

FAT12 Data Recovery

FAT12 is predominantly used for the storage of data files on floppy diskettes. The most common problems requiring data recovery are therefore related to recording quality issues and physical problems. Whilst diskettes are remarkably robust considering what they are, they are physically frail and used in low cost drives that are primitive in their operation. It is unusual to find a set of diskettes that do not contain one or two with errors. What is sometimes surprising is how much data is still stored on diskettes. Archives of small data files that predate the advent of CD and DVD are often kept, then forgotten about, only for the data to be needed some 10-20 years later and then a data recovery is required to get hold of the data.

FAT16 Hard Drive Data Recovery

One of the biggest problems with FAT16 is when there is allocation data loss. The second copy of the FAT does give a degree of protection but the proximity of the two copies to each other is a significant vulnerability. Minor problems within the file system can be circumvented, although not always without some loss of data, but if both copies of the allocation table have been overwritten or wiped then it is only by working through the data contents that a data recovery can be made.

FAT32 Hard Drive Data Recovery

FAT32, whilst less rigid in its structures than FAT, is still vulnerable to problems resulting from data being written to the allocation table area, and to file deletion. Whereas NTFS does not erase the allocation information for a file that is deleted, under FAT32 all allocation information is removed when a file is deleted and so data recovery is reliant on either the file system not being fragmented, or on the data to be recovered being identifiable by its contents. When there is damage to both copies of the allocation table the problem is dramatically more severe, though if enough of the FAT remains then the remaining data from both copies can be combined to provide as complete a copy of the allocation information as is possible.

Altirium's hard disk drive recovery specialists have a wealth of experience in dealing with all versions of FAT file systems and with the recovery of data by salvaging the data for specified file types, and dealing with FAT damage and corruption.

Last Updated (Friday, 19 June 2009 08:00)

QMD 9001 logo
ISO9001:2008 Certified