Firewire can transmit video or connect drives for data transfer. Theoretically you can connect 63 firewire devices to a system. However, my practical experience is a maximum of 4 drives on each firewire port.
A video device communicates differently than a drive. We have seen issues when you connect a firewire drive, and a video device to the same card. Both Aja and Avid recommend using an additional firewire cards.
This diagram shows how devices communicate on a Mac Pro. We see that the Firewire 400 and 800 ports on the front and back of the Macintosh connect to the same internal Firewire chip.
In my experience, if you connect a video device to any of these ports on the front or back, and a firewire drive you will have a potential for conflicts. This is displayed as dropped frames, the deck not responding, the capture aborting, or video not being able to control the video device properly.
The best way to work around this is to either add an additional firewire card to the system.
Firewire cards we have had success with:
These are best practices to ensure you have success every time you work with your video equipment.