If you want to do this correctly, let me tell you about a better solution.
A two SATA port Express card and two SATA drives with replacable trays.
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www.thefinalcutstore.com]
I am using this to capture 1080i off of an HVX200 camera in real time.
Firewire 400 or 800 have design limits on the throughput that can not be exceeded even with a RAID speed volume set. This card set can deliver better than average RAID speeds.
Just an alternative idea.