What is a lost cluster?
Answer: A lost cluster, or file fragment, is a small amount of data that does not belong to any file. It does not have a file name and is not mapped to any directory. Because they are invalid files, these enigmatic pieces of data can be deleted with no harm done to the valid data on your hard disk. Lost clusters are usually created when your computer is shut down improperly (i.e. pulling out the plug while some files are copying), or when you eject or disconnect a removable drive before unmounting it.