Jace05 and burgersplus.
I am sorry the instruction I gave haven't worked for you, I did make it quite clear in an earlier post that I have never done this myself. I don't have Linux on an external drive and don't have any reason to do that, I am simply applying the same techniques to external drives that I have successfully employed many times on internal ones. My logic for doing that is simply 'A drive is a drive is a drive' as far as I know, be it internal or external. From what you have said maybe that isn't the case.
However, if ,bearing the above in mind, you are still willing to experiment, this is what I would do next (I am assuming here that you have already set your bios boot order to 1)cdrom 2) usb 3) hard drive).
With your external drive plugged in boot from the live cd then open a terminal and run:
answer will be something (but not exactly) like
In other words there should be grub stage 1 files on hd1 which should be your external hard drive (NB If you have more than 1 internal drive beware as your external drive may not be hd1). Bear in mind also that disks and partitions are numbered from the number 0 so (hd1,1) is the second partition on your second drive. Bear in mind again that the numbers I have used here are for illustration only, yours will probably be different.
To continue with my example, where the find command has found stage 1 files on (hd1,1) so I now want to make sure that grub is installed on the same drive I would run:
to exit the grub prompt.
Then remove the live cd and reboot with the external drive plugged in and I hope it will boot Mint, but I can't be sure because as I said above I have never done this myself.
I wish you both luck anyway.
Oh and bear in mind what Fred has said, mixing my instructions and his at the same time will definitely
And btw if this doesn't work then I am completely out of ideas, sorry!
Fujitsu Lifebook AH532. Intel i5 processor, 6Gb ram, Intel HD3000 graphics, Intel Audio/wifi. Realtek RTL8111/8168B Ethernet.Lubuntu 13.10,Ubuntu12.10 (Unity), Mint16 (Cinnamon), Manjaro (Xfce).