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Upgrading and Repairing PCs |
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FAT File System ErrorsFile system errors can, of course, occur because of hardware problems, but you are more likely to see them result from software crashes and improper system handling. Turning off a system without shutting down Windows properly, for example, can result in errors that cause clusters to be incorrectly listed as in use when they are not. Some of the most common file system errors that occur on FAT partitions are described in the following sections. Lost ClustersProbably the most common file system error, lost clusters are clusters the FAT designates as being in use when they actually are not. Most often caused by an interruption of a file system process due to an application crash or a system shutdown, for example, the FAT entry of a lost cluster might contain a reference to a subsequent cluster. However, the FAT chain stemming from the directory entry has been broken somewhere along the line. Lost clusters appear in the file structure as shown in Table 24.28.
The operating system sees a valid chain of clusters in the FAT but no corresponding directory entry to back it up. Programs that are terminated before they can close their open files typically cause this. The operating system usually modifies the FAT chain as the file is written, and the final step when closing is to create a matching directory entry. If the system is interrupted before the file is closed (such as by shutting down the system improperly), lost clusters are the result. Disk repair programs check for lost clusters by tracing the FAT chain for each file and subdirectory in the partition and building a facsimile of the FAT in memory. After compiling a list of all the FAT entries that indicate properly allocated clusters, the program compares this facsimile with the actual FAT. Any entries denoting allocated clusters in the real FAT that do not appear in the facsimile are lost clusters because they are not part of a valid FAT chain. The utility typically gives you the opportunity to save the data in the lost clusters as a file before it changes the FAT entries to show them as unallocated clusters. If your system crashed or lost power while you were working with a word processor data file, for example, you might be able to retrieve text from the lost clusters in this way. When left unrepaired, lost clusters are unavailable for use by the system, reducing the storage capacity of your drive. The typical choices you have for correcting lost clusters are to assign them a made-up name or zero out the FAT entries. If you assign them a name, you can at least look at the entries as a valid file and then delete the file if you find it useless. The CHKDSK and SCANDISK programs are designed to fix lost clusters by assigning them names starting with FILE0001.CHK. If more than one lost chain exists, sequential numbers following the first one are used. The lost clusters shown earlier could be corrected by CHKDSK or SCANDISK as shown in Table 24.29.
As you can see, a new entry was created to match the FAT entries. The name is made up because there is no way for the repair utility to know what the original name of the file might have been. Cross-Linked FilesCross-linked files occur when two directory entries improperly reference the same cluster in their Link to Start Cluster fields. The result is that each file uses the same FAT chain. Because the clusters can store data from only one file, working with one of the two files can inadvertently overwrite the other file's data. Cross-linked files would appear in the file structure as shown in Table 24.30.
In this case, two files claim ownership of clusters 1002 and 1003, so these files are said to be cross-linked on 1002. When a situation such as this arises, one of the files typically is valid and the other is corrupt, being that only one actual given set of data can occupy a given cluster. The normal repair is to copy both files involved to new names, which duplicates their data separately in another area of the disk, and then delete all the cross-linked files. Deleting them all is important because by deleting only one of them, the FAT chain is zeroed, which further damages the other entries. Then, you can examine the files you copied to determine which one is good and which is corrupt. Detecting cross-linked files is a relatively easy task for a disk repair utility because it must examine only the partition's directory entries and not the file clusters themselves. However, by the time the utility detects the error, the data from one of the two files is probably already lost—although you might be able to recover parts of it from lost clusters. Invalid Files or DirectoriesSometimes the information in a directory entry for a file or subdirectory can be corrupted to the point at which the entry is not just erroneous (as in cross-linked files) but invalid. The entry might have a cluster or date reference that is invalid, or it might violate the rules for the entry format in some other way. In most cases, disk repair software can correct these problems, permitting access to the file. FAT ErrorsAs discussed earlier, accessing its duplicate copy can sometimes repair a corrupted FAT. Disk repair utilities typically rely on this technique to restore a damaged FAT to its original state, as long as the mirroring process has not corrupted the copy. FAT32 tables are more likely to be repairable because their more advanced mirroring capabilities make the copy less likely to be corrupted. An example of a damaged FAT might appear to the operating system as shown in Table 24.31.
This single error would cause multiple problems to appear. The file USCONST.TXT would now come up as having an allocation error—in which the size in the directory no longer matches the number of clusters in the FAT chain. The file would end after cluster 1001 in this example, and the rest of the data would be missing if you loaded this file for viewing. Also, two lost clusters would exist; that is, 1002 and 1003 appear to have no directory entry that owns them. When multiple problems such as these appear, a single incident of damage is often the cause. The repair in this case could involve copying the data from the backup FAT back to the primary FAT, but in most cases, the backup is similarly damaged. Normal utilities would truncate the file and create an entry for a second file out of the lost clusters. You would have to figure out yourself that they really belong together. This is where having knowledge in data recovery can help over using automated utilities that can't think for themselves. 
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