A hard drive uses a File Allocation Table (FAT) that keeps track of where a file is located, how large it is, beginning and ending sectors, etc. When you delete a file a single bit in the FAT table is changed to make the file not visable and to make the area useable for something else. This is why if you use an "undelete" program you can restore a file many times. It just changes that bit back. Since the area on the drive was available for writing new data, if you've added and deleted new data since the deletion your recovered file may be corrupt.
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Howie & Mary Ann Pfeiffer 
Lebanon, PA but primarily from Hackensack Area, NJ ,
Village of Gilchrist
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