FW audio interface glitches when using ext FW HD

Hi there,
I am on an Imac and use an Maudio 1814 firewire soundcard. My Imac model has 2 FW ports. One 400 and a 2nd 800. When i plug an ext FW HD on the 2nd port and try to read or write i get glitches on my speakers…I suspect this happens because both ports use the same bus…Not sure but thats what i ve heard.

Now i need to get an interface cos the one i am using is not mine and if there is such issue i ll have to get a USB one as i need to use my HD at FW800 speed.
And also, could i face similar problems with a USB audio interface and a USB ext HD?

Anyone can comment on this?