DualBoot Vista-Fedora (1 OS on externall USB HDD)

[CompSol]

Hi, I`ve been looking for a way to get working dual boot, no success so far.
I have Vista installed on my PC, no troubles at all, booting fine.
A while ago I installed Fedora on a new externall USB HDD 500GB (Samsung S2). Installation OK, there is a bootsector, menu.lst as well. My BIOS can recognize HDD, and I can boot from USB stick. But something doesn`t work, or I am missing something, I am no expert, just following all the guides I can find on the net (so often incomplete or messy), I just can not boot from USB HDD. I`ve tried using EasyBCD and GAG software, no luck.
There must be something specific to booting from USB HDD perhaps. Or is it a problem that I have two NTFS partitions on the HDD too (no OS though, just a storage)? There`s a lot of experts with guides on how to install and dual boot two different OS from one HDD... But what about one OS being installed on externall USB HDD?

 

[CompSol]

It works now

In case it might be helpful to somebody, here is what I`ve done to get it work.
It seemed to me that the main problem was in the mbr of the disc. Probably it can be placed just to the first sector of the HDD, so there has to be space available in the very beginning of the disc. Since there was NTFS partition with no OS as the first partition, the external HDD was without mbr being present actually, and thus placing a boot sector anywhere was pointless. There might be a better explanation, I`m just a beginner.
So, I resized and moved the first NTFS partition and made a free space before, so the first became an unallocated space on the HDD. Into that I installed Fedora, configured the /boot to be installed to the very first sector of the HDD, GRUB installed to the mbr in my case (since it`s the first and the only one OS on that HDD), and it works just fine. Now I can boot Fedora two ways without troubles. First when I set up BIOS to boot the external first, it boots straight away. Or I can set up BIOS to boot the internal first, and during BIOS start up press F8 (in my case) to choose where to boot from, select the external from the list and it works now.
All that was not possible before without the mbr being present and installed in the first sector of the HDD.
I don`t want to go into any more details so not to confuse anybody with a bit different environment, it can make a difference possibly. So anybody with a proper technical explanation/details welcome to post it here.