I have this issue with all the Firewire ports on my Mac Pro. All the drives I connect to the Firewire will have write issues which result in drive crashing (in the case of my Drobo) or very slow write speed (less than half of normal). I have already gotten a replacement Drobo and tested a new enclosure I just bought. No dice. I cannot duplicate this issue on my MacBook Pro. I have tried many enclosures, different cables, etc. All have this issue on the Mac Pro only. USB works without any issues and eSATA (via a expansion card) works as well. Apple Hardware Test was run for a few hours and everything passed. New account created. Issue still persists. I don't have a car, so taking my machine to the store is a tedious task. I'd like to avoid that if possible. Anyone have thoughts on this? Thanks!