FAT File System Errors

Although the FAT file system is very efficient given its age, its overall design and how it allocates disk space and chains file together provides ample opportunity for errors to occur over time. Most times they are quite unexpected and always tend to occur at the worst possible moment. Bear in mind that we are referring to errors in the hard disks logical structures, physical disk errors, bad sectors, etcetera. Fortunately, most of these errors can be detected, and most cases repaired, by using standard disk error-checking programs that analyze the file system's integrity. Programs such as Microsoft's SCANDISK have been available in one form or another since the release of MS-DOS and continue through the recent release of Windows XP. There are also third-party programs such as Norton's Disk Doctor and other Norton disk tools.

We recommend that you scan you hard disk for errors at least once per week if you are an average user, and more often if you are a power or advanced user.

There are a number of causes for file system errors, and they include:

• Disk corruption emanating from physical hardware problems.
• Hardware resource conflicts.
• Bad drivers
• Software issues such as operating system and/or program crashes. These are the more prominent as crashes often deposit data into clusters and the fail to close and mark the location of either or both the preceding or next cluster.
• A loss of power to a computer running Windows will often result in one or more file system errors due to files not being closed properly.
• Improperly exiting the Windows operating system or shutting your computer off improperly (not closing Windows properly) will often cause data problems. Beginning with Windows 95, Windows will automatically scan the disk for errors when starting, if it detects and improper shutdown.

The the most common errors encountered on a FAT disk are:

• Lost Clusters:
  Every user will, from time to time, experience "lost clusters", whether they realize it and fix it themselves, or be plagued by the vagaries they cause if they don't realize there's a problem. Lost clusters are simply those that are marked in the FAT as being in use, but, in fact, not in use and which the system cannot link to any file. Every file that is saved on your hard disk consists of a series of clusters that can be traced by starting with the directory entry in the FAT and following the link list of clusters to the end of the file. Disk checking programs do exactly this to verify file structures. They scan the entire disk volume for lost clusters using a procedure similar to the following:
  
  
  1. Create a copy in memory of the FAT, noting all of the clusters marked as in use.
     
     
  2. Beginning with the root directory, they trace through the clusters used by each file and mark them in accordance with the FAT if, and only if, they are connected to a file. The procedure continues in the same manner for all the subdirectories of the root directory, and then their subdirectories, and so forth.
     
     
  3. At the conclusion of the checking and verification procedure, every cluster that is marked in the FAT as being occupied by a file will be accounted for. Any files found to be in use, but associated with a file will be marked as "orphaned or lost clusters". Most times these lost clusters won't be deleted, but rather saved and you will see them listed by the program as lost clusters in the event you might want to retrieve any data that may be within them.
  
  
  

  Again, lost clusters are usually the result of interrupted file activity of some sort. A program may attempt to allocate clusters to a file it is building, and if the file is not properly finished and closed, the clusters never get correctly linked to a file name. This usually happens when computers are abruptly turned off without the operating system being properly shut down. The program that detects lost clusters will usually give you the choice of clearing them by marking them as "available" and returning them to the pool of free clusters, or saving them as a file. In the latter, the program generates an artificial file name and links the lost clusters to that name, so that a real file is formed. In most cases, these files are damaged in some way, but often you can see what this orphaned data was and in some cases, recover it.

• Cross-Linked Files:
  This is one of the more unusual vagaries of this type of file system. Essentially, two clusters from different files ends up pointing to a single cluster, but the same one. Alternately, one cluster in the middle of two or more cluster chains may point to the same place. Obviously this is a problem, as the same cluster cannot be part of more than one file. Each time you save data to either of the cross-linked files, you are overwriting all or part of the data in the other one. The only solution to this problem is to make new copies of each of the affected files, however in most cases you will lose the contents of one or the other of the files. Usually, by the time you discover the problem, the damage has already been done. Quite often, both files will be lost and you will need to restore them from a backup if you have one.



• Invalid Files or Directories:
  One of the more daunting problems of this file system is that of an invalid file or directory. This usually occurs when a disks internal file structures become damaged to the extent that some entries are no longer following the FAT file structure at all. A good example would be a directory that doesn't have a pointer to its parent directory, or a file that has an invalid starting cluster. One of the more common problems is the assignment of invalid dates or times by cheap software. These problems, however, can usually be fixed by disk scanning software.



• Allocation or FAT Errors:
  Occasionally the entries in the FAT can become corrupted or set to invalid values. Again, most disk verification utilities will detect and correct these types of problems.

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