I’ve had problems with those large drives over firewire 800 as well. For me it was as simple as hooking them up via firewire 400, but I only had 3 drives going. The other thing you can try instead of restarting all the time is opening the disk utility and trying to mount the drives from there (they also might need a simple repair done which can be done in the disk utility as well). It sounds like you have a few terabytes of storage there, you should probably think about getting a RAID, it will save you a lot of headaches.