The presonus has 2 ports most likely just because Firewire devices can be daisy chained. I'm not sure if I'd record directly to a Firewire drive - but I haven't tried it - so you might have sufficient BW there. I have Presonus Mobile as well (just got it about a month ago) - but just record to the internal drive, that's the plan anyway. If I need to, I'll just move things to an archive drive. I'm sure folks on the presonus forums can give you an idea of how they set things up. I'm just a recording infant - so take my advice from that perspective.

I have 2 FW800 drives from these guys Performance Upgrades; FireWire USB SATA Storage; Memory, more at OWC I don't think you can go wrong.