Going through firewire has been problematic for me with the P2s I kept getting the same kind of error message you’re getting when I was trying to capture from the camera into FCP.
After trying everything I could think of, I finally switched firewire cables and voila, the clips imported without a problem. Have you tried changing out the firewire cable between your Mac and the G-Raid or maybe trying firewire 400 (if the G-Raid has the connection.)
If it’s not difficult, why do it?