I googled around and can't find anything obvious.

sometimes when I plug a FireWire drive into my macpro, another fw drive disappear? I have as many as 6 external drives which include both usb & fw 400 & 800's.

anyone else have this issue? anyone find a way to not lose a drive?

this is most likely a drive chipset issue ... not all firewire chipsets are created equal (especially not the cheap ones).
i have a few firewire drives that I use now and again, some of them are clearly better than others ... amongst them I have 2 identical cheap firewire boxes, and they will not mount simultaneously on the same FW bus, its one or the other.