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 This program is useful to rescue data in case of I/O errors, because it does not necessarily abort or truncate the output. This is why you need to use this program and not the dd command. I have recovered much data from many disks (CD/hard disk/software raid) over the years using GNU ddrescue on Linux. I highly recommend this tool to Linux sysadmins.