I think you can use 'SLAX' for making your external hard disk bootable. Slax is actually a flash drive OS but it can be installed on any hard disk also.
What it does is it will edit your HDD's or flash drive's MBR to make it bootable. The process is just simple, you have to download the slak package from www.slax.org
and unzip to your respective drive. It will contain two folders, 'boot' and 'slax' and in the boot folder there will be scripts 'bootinst.sh' for Linux and 'bootinst.bat' for windows. just run it and it will make the current drive bootable with the slax os installed.
You can also add other kernals by eiditing its respective boot files which I think you will be more expert than me!!!!!!
Please try this and let me know if it worked!!!