Does this look like the correct cable?
http://www.xpcgear.com/cable48.html
At least one of the support forum posts suggested that it might be an EPS to Molex cable.
I see that Matrox responded in the thread and suggested that the user contact their technical support.
http://forum.matrox.com/rtx2/viewtopic.php?t=7575&highlight=cable
If your card is less than 3 years old, they might send you a cable.
I would recommend that you verify that the cable is wired the same as a standard EPS cable before powering up the system.
Even without a schematic, a skilled electronics technician should be able to at least verify that the ground leads in the EPS cable match up with the ground pins on the card.
Chas
I did find a few more posts in the forum where people were asking how to build their own cables. In all instances, the moderator told them to contact customer support or to check with their dealer for a replacement cable. To me, this suggests that the cable may not be wired the same as a standard EPS cable.
Another note was the recommendation that the power not be shared with another device on the same cable. I suspect that electrical noise is the main problem. But, voltage drop across multiple connectors on a drive power cable could also be the concern.
Chas