I'd try the grub re-install to the MBR of each drive first ... I think that should work and give you what you want.

You will need to follow devils caspers suggestion if either:-
1. your BIOS does not support boot from USB
or
2. BIOS limits allows only boot from partition less than 1024 cyclinder limit.

I had assumed neither was an issue and you were able to boot Linux on your internal hard drive before you did the external hard drive install.