[CentOS] Booting from a USB hard drive?

I'm trying to learn Asterisk (specifically Trixbox) and, as you
probably know, it runs on CentOS. I want to run it on my laptop and
thought that I could install it to an external USB hard drive, like
I've done with Fedora 12 and Linux Mint 8 (I like to keep my small
CentOS/Windows dual booting hard drive in the laptop's hard drive
slot). But it won't work. It kept giving me hard drive errors when I
tried to install it to the USB drive, so I installed it in the regular
hard drive slot in the laptop -- where it works fine. But when I move
it to the USB enclosure, it starts to boot, but can't mount the
partitions and goes into kernel panic. I checked this with my CentOS
hard drive and it does the same. I'm guessing the only reason Linux
Mint works is because it refers to all partitions as "sda" instead of
"hda" -- so it doesn't know it's in the wrong slot (just a guess).

Is there an easy fix for this? Is there some configuration file that I
can change entries from "hda" to "sda" so the partitions will mount or
is it more involved than that?

Thanks for any pointers.

-- 
RonB -- Using CentOS 5.5