Cannot option boot when Matrox Triple Head 2 Go Digital Edition (with USB Cable) is attached.

OSX Yosemite (most current version) - I have a Monitor and TripleHead2Go (Matrox) Digital Edition hooked up to MAC PRO (5,1) ATI 5870 Video Card. The TH2G requires a USB cable for powering the device. I have 3 separate SSD/HDD drives I use for booting to 3 separate environments so I use the Option Boot process to boot to these drives and select the one I want for the work to be done. However, when the TH2G USB cable is connected, it will not run the boot process - stalls as long as I have the Option key depressed and when released, it boots to the default drive, thereby bypassing the option to select a non-default drive. If I remove the USB cable before booting, it works fine and after I've selected the correct drive to boot, I plug the USB cable back in and everything works as advertised. I assume that this USB connection conflicts OS X as to whether it is a bootable drive or not?

Any ideas to get Option Boot process to work with this configuration without having to unplug/re-plug the USB power cable from TH2G? I contacted Matrox support and they were no help. I also tried to use a USB power connection from a separate source (vice the Mac Pro) but with no luck.