when you installed Mandriva it most likely removed the 'Boot' flag from the internal HD
what you would need to do if that is the case is (and this is the only way I know if anyone else knows an easier way follow it) DL GParted and add the 'Boot' flag back to the original HD, you should then be able to boot from that HD, then edit your boot.ini and add the external HD/OS as an option
here is a link to a site to help if you don't know
how to edit boot.ini
I am still a newb and not sure if I offer the best and easiest way to fix your issue
I hope this is of some help to you