Partition and Raid Setup

First a Raid 1 (mirror) has to have matching partitions. If you RAID 60 and 80 GB partitions, if DU will even let you do it, the last 20 GB won't be usable. You would have to make the 20 GB another (third) partition. Second if you create a RAID using the external firewire drive, you will not be able to run without the firewire drive unless you break the mirror, in which case you will have the overhead of syncing the firewire partition to the mirror when you connect it the next time, otherwise the value of the mirror is nil. Third, by creating a mirror between an internal drive and an external firewire drive, you are going to take a performance hit, that's beyond the hit you'll take for having to do 2 physical writes (2 drives) for each logical write. Why? The internal drive will be faster, but it will have to wait for the firewire drive on all write operations. Lastly, you'll take another hit when using you're non-mirrored FCP partition, since it shares the same physical drive as your RAID. In short, this doesn't make a whole lot of sense to me. Why not just backup your internal drive to the firewire drive periodically and be done with it? I recommend CarbonCopyCloner.