Thought I would share this. I had a 500GB samsung external drive that failed and could not be read by any OS I tried. On my UM system under GPARTED the error was simply, “i/o read error”, I even tried rescatux to access the disk, but no luck…
After some determined searching I was pointed to the ddrescue tool (http://www.gnu.org/software/ddrescue/ddrescue.html), turns out a great command line unix tool. I managed to recover all the data from the drive. I followed the guide at https://www.technibble.com/guide-using-ddrescue-recover-data/ and information at http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html and things went well, although the recovery did take a full day to create the disk image. I also had to use the mount offset instructions, but it worked perfectly.
If you are having trouble rescuing data from a disk, do look at this tool, it is very versatile and saved the day for me!
Hope this helps someone!