There’s only one FW buss in a Mac. There may be multiple ports, but only one controller. That means that sometimes with two or more devices connected, the buss will choke. While this happens more often when two different devices like a camera and drive are connected, I’ve also seen it happen with just two drives. It probably has to do with the FW chipsets being used somehow not being compatible. Try unhooking the drives one-at-a-time and see if the problem clears up. Maybe hooking them up in a different order may help. If you can move the files using the Finder, but not in FCP, then trash FCP prefs and repair permissions. Perhaps, that will help.
John
MacPro 8-core 2.8GHz 8 GB RAM OS 10.5.8 QT7.6.4 Kona 3 Dual Cinema 23 ATI Radeon HD 3870, 24″ TV-Logic Monitor, ATTO ExpressSAS R380 RAID Adapter, PDE enclosure with 8-drive 6TB RAID 5
FCS 3 (FCP 7.0.2, Motion 4.0.2, Comp 3.5.2, DVDSP 4.2.2, Color 1.5.2)
Pro Tools HD w SYNC IO & 192 Digital I/O, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec Monitors, PrimaLT ISDN