If the external drive is the internal drive that you connect it to the hard drive enclosures, then you can do it but please to note they will clone the whole thing into that drive and all the old data will gone on that hard drive.
You can clone to a SSD too, if you don't have either WD or Seagate/Maxtor drive, then you can use the " EaseUS Todo Backup Free "
http://www.easeus.com/backup-software/tb-free.html
Or you can try this 1) clone the whole thing into one SSD or internal HDD.2) To create an image to the external drive and make a system repair disc. In this way you have two copies of your OS. So the SSD is dead, you can either put in the other SSD or insert the system repair disc and connect the external drive, boot the PC follow the step to step to restore the OS.