If the data is important I would highly recommend sending it to a specalised data recovery company, they have special hardware/software designed specifically to deal with failing hard drives. Digital Recovery (www.digitalrecovery.co.nz) are well priced and have all the gear to deal with this. If you data is not particularly important you could try removing it from the external case and imaging it in a Linux machine using dd_rescue. Download Ubuntu and install dd_rescue (sudo apt-get install dd_rescue) and have another blank drive installed then image the disk like this "dd_rescue /dev/sda dev/sdd, where sda is the faulty drive and sdd is the blank one, do not get these mixed up or you will overwrite your data.