Are the two drives daisy chained together? I have a FW HD and DVD drive daisy-chained and if I burn a DVD from the FW HD, the HD sometimes unmounts. Not often though but it does hang everything up. You usually don't have to reboot btw. If you just pull out the FW cable, it frees OS X up - you get a warning but I've done it loads of times without a problem. Don't do this if the drives are bus powered.
I copy between encrypted disks a lot and it works ok. Could it be something to do with using FW800 maybe? I only have FW400 drives. Also, are they sparse images or fixed size?