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If you're using a Microsoft Windows operating system and your hard drive is formatted using one of the FAT systems (FAT16 or FAT32), you should run ScanDisk every so often to help maintain your drives.


              Root Directory Table
 __________________________________________________
|filename|ext|  date  | time |info|starting cluster|   
|________|___|________|______|____|________________|
|__nate__|jpg|11-15-01|11:48A|____|_______1________|
|________|___|________|______|____|________________|
|________|___|________|______|____|________________|
|________|___|________|______|____|________________|

File Allocation Table
   ______________
  |Cluster|status|
  |_______|______|
  |___1___|___2__|
  |___2___|___5__|
  |___3___|___4__|
  |___4___|___6__|
  |___5___|__eof_|
  |___6___|__eof_|
  |/\/\/\/\/\/\/\| continues...


The File Allocation Table (FAT) and the Root Directory Table work together to track files on a hard drive. When a file is deleted, when a system crashes, or when a power fluctuation causes data to be mis-entered on the drive, you can get items called lost clusters. A cluster is a block that Windows can place information in. Most programs and data take up more than one cluster, so the system must track where one cluster ends and the next one starts. If you have not defragmented your drive, the data can be spread over a large area on your drive.

Looking at the firat table, you will see that there is a file on the hard drive called nate.jpg. It starts in cluster 1. Looking at the second table, you will see at the end of cluster 1 it directs you to go to cluster 2 for more data. This is called chaining, as the data is chained together to create one file. At the end of cluster 2, it directs you to cluster 5. At the end of cluster 5 you get the eof, which means end of file. It looks like the nate.jpg file is just fine. Now, look at cluster 3 on second table. There is some data in that cluster, and it is chained to cluster 4, which is chained to cluster 6, which has an eof. Unfortunately, there is no entry in the Root Directory Table concerning a file that starts on cluster 3. Maybe it was erased, maybe you were copying WonkoDSane.jpg to your hard drive when it made Windows crash, but there is information located there that is unusable.

Having these lost clusters and chains takes up space on your hard drive. If there is anything but zeros in that cluster, it is off-limits to Windows. If you erased nate.jpg, the clusters would have the data changed to zeros so the system can re-use that cluster. Because there is no entry in the Root Directory Table, you cannot erase the data. That's where ScanDisk comes in. It checks where all the clusters start and stop, and marks any non-chained cluster. ScanDisk then asks you if you want to delete the data in these lost clusters or convert them to a file for you to look at. Erasing them just drops a bunch of 0's into the cluster. To convert the lost clusters to a file that you can look at, all Scandisk does is this:


              Root Directory Table
 __________________________________________________
|filename|ext|  date  | time |info|starting cluster|   
|________|___|________|______|____|________________|
|__nate__|jpg|11-15-01|11:48A|____|_______1________|
|file0001|chk|02-15-01|11:45A|____|_______3________|
|________|___|________|______|____|________________|
|________|___|________|______|____|________________|

File Allocation Table
   ______________
  |Cluster|status|
  |_______|______|
  |___1___|___2__|
  |___2___|___5__|
  |___3___|___4__|
  |___4___|___6__|
  |___5___|__eof_|
  |___6___|__eof_|
  |/\/\/\/\/\/\/\| continues...


Now, there is a file called file0001.chk that you can look at or delete. It starts at cluster 3, and the clusters are chained just as before. If you delete file0001.chk, clusters 3, 4 and 6 will be zeroized.

So, running ScanDisk or other disk maintenance program frees up space so you can download the homenode pics of all the cute folks on Everything2.

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