Firewire drive and camera conflicts

Hello.

I'm using a newer Mac tower with an Intel 2x2 GHz Dual-Core Xeon. OS 10.5.1.

I'm trying to use two external hard drives (Lacie and WD). When I plug them both into fire wire (one in the back and one in the front) I can't see them both.

Also, When I try to plug one of the hard drives into firewire in the back and a camera in the front, the computer won't see the camera.

Any ideas? Is my machine bogus or am I trying to use firewire wrong?

Should I just be using USB2 for my hard drivers and reserving firewire for my camera?

Thanks in advance!