As long as the BIOS sees the external hard drive it's a piece of cake. Just stick the install disk in the CD/DVD drive, make sure the PC boots from there first and then follow the instructions. I installed it that way on my work laptop and it was just fine.
I'm not sure whether this is a laptop or desktop you're putting it on. If you're super paranoid about overwriting data on your existing hardrive, it's easy enough to pull the cable out of a SATA or IDE drive on a desktop while you do the install. I can't remember exactly what options I was given when installing it on my work laptop, but I managed to not overwrite all my data - which IT would have been severely nonplussed about .
After the install, you need to need to make sure that booting from a USB port has higher priority than your internal drive, that way as long as the USB drive is plugged in, it will just boot to Mandriva.
If it's a laptop, make sure you don't do what I kept doing: forget the little USB drive was hanging out of the back, stand up and have it fall to the floor!
Am so glad to have my own Linux laptop now!
