1. A method for securely erasing data from a computer-readable host storage medium in a host computer system which has a host operating system, and in which the host storage medium stores separately read/writable files in at least one used space, the method comprising: selecting at least one of the files as a target file for secure erasure; selecting a set of replacement data from the used space of the host storage medium that is representative of at least one source file in the used space; replacing the target file on the host storage medium with the replacement data; fragmenting selected ones of the source files over locations on the storage medium containing the target file; and, thereafter defragmenting the source file fragments.