One internal S-ATA drive should be way faster compared to your firewire disk.
Connect your LaCie to you MacPro and see if it works.
Or Try your Macintosh HD.
This way you can check out for sure if it’s a system or harddisk issue.
You could also try a AJA or Decklink speedtest to make sure the drive is getting enough speed.
You should be able to do at least one stream HQ on a internal S-ATA drive.
Does a lighter codec work without problems? Just for testing purposes.