I take it that the Rosewill card is getting detected at boot?
I read it that the fire wire devices are hot swappable but I may be wrong, and I believe that the card needs to be connected at system boot so that the BIOS can register it allowing the OS to load the correct drivers.
We have conflicting info on the Siig
I am going to stand by my theory that the card is not fully Windows 7 compatible unfortunately because it works (sometimes) I think it is just being pushed as Windows 7 compatible. It may be worthwhile to attempt to install it (uninstall old drivers first) with the XP drivers in compatibility mode; Compatibility Mode
If this does not work Go through this tutorial USB Driver - General Fix for Problems
You can read the lines at the top but ignore the bit about your mouse as you are going to do this for fire wire.
Follow the tutorial through, but make the following changes;
the word USB
with the word Firewire
and when it comes to uninstalling drivers uninstall ALL Firewire drivers. Not as written just the faded entries.
IT IS IMPORTANT TO UNINSTALL ALL FIREWIRE DRIVERS
The tutorial is quite long and it is important to do it right, take your time and go through it one line at a time until you are finished. It may help to print it out, so you are not constantly swapping open windows.
I cannot guarantee that the tutorial will work, but when done properly it will also do no harm.