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A computer disk stores information in a series of chunks known as sectors, each typically 512 bytes long. Each sector has a number that serves as its address on the disk. A file on the disk is split across a number of sectors. These sectors may be located together but might be scattered across the disk if there is not enough contiguous space in one spot. When a file is deleted, the information stored in the individual sectors is not erased, because the erasure process consists of overwriting the sector and is relatively slow. It is significantly faster to overwrite the sector by rewriting it with new data only when it is needed for some other file. So when a user "deletes" a file, the directory entry for the file is either removed or marked as deleted by changing the first letter of the file name to a special character.A deleted file can be recovered if the index information and sectors have not yet been overwritten. The chances of this are better if little computer activity has occurred since the file was deleted. Microsoft MSDOS and Windows changes the directory entries by changing the first letter of the filename. File undelete software will scan the directory for filenames that have the special character that signifies "deletion" and present a menu of files to recover. When the accidentally deleted file is chosen, the directory entry is corrected and the file reappears. The user often has to supply the first letter of the filename for restoration to occur.When you lose data on a hard drive, a sector editor allows you to view data almost everywhere on the drive.