Sounds like your concern is hard-drive failure. Your best option is to shoot to RAIDED drives in your computer. Get two hard drives of equal size and mount them in slots 2 and 3 of your macpro hard drive sleds. Use Disk Utility to create a RAID 1 array and put your capture folders on the RAID drive when you are shooting. It will write the files simultaneously on both drives and if one dies, the other takes over automatically so you've lost no files. Also since the drives do not hold OSX files, they can be easily mounted in another machine if the system itself dies.
I am in agreement with Erik when it comes to running multiple items over the same fw bus.
If you must use an external drive, Chronosync and SynchronizeX Pro are great options, but will slow down your system while shooting.