A group of disksectors. The operating system assigns a unique number to each cluster and then keeps track of files according to which clusters they use. Occasionally, the operating system marks a cluster as being used even though it is not assigned to any file. This is called a lost cluster. You can free up disk space by reassigning lost clusters, but you should first make sure that the clusters do not, in fact, contain valuable data. In DOS and Windows, you can find lost clusters with the ScanDisk utility.
Hard Disk Logical Structures and File Systems Describes "standard" file systems used on PCs, with particular focus on the FAT file system and its variants. This page is from "The PC Guide."
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