GNU ddrescue is a program that copies data from one file or block device (hard disk, cd/dvd-rom, etc) to another, it is a tool to help you to save data from crashed partition i.e. it is a data recovery tool. It tries to read and if it fails it will go on with the next sectors, where tools like dd will fail. If the copying process is interrupted by the user it is possible to continue at any position later. It can copy backwards.