Can't erase unmountable external HD
You can try entering in Single User Mode and do an fsck on the hard drive. 

To enter single user mode, turn on your computer and immediately after press the Command s. This will load you to a terminal looking screen. Let it run its pre-checks. 

After it is done there should be two commands to run:


a command to mount the drive 
a command to run fsck on the drive. 


Do the fsck first. This will make sure that at least the hierarchy of the OS file system is ok. Use the mount command just to see if you can mount the Drive. 

If it fails, there could be a hardware issue, a cable or failing hard drive. If it does not fail and passes everything, reboot. See if it works normally.

I believe the following command is for the fsck

/sbin/fsck -fy

and the following for mounting

/sbin/mount -uw /

Single user mode will display the correct commands.

Once you are done with Single User mode type reboot and hit Return

And this will boot you up normally.