You may want to try using a different disk utility such as Disk Warrior to see if the problem lies with the directories on the drives. It does seem strange though, that two drives with different interfaces should refuse to mount.
Try this, even though it sounds a little strange. Remove the Firewire cable from the Firewire drive. Turn off the power to the drive. Wait for 10 seconds or so and then turn the power back on and plug in the Firewire cable. Let's see if we can get the system to look at the drives again. If that works, try the same with the USB device.

Hope this helps

P