I have both a Plextor Premium and Premium II in an external enclosure (can’t remember the name but very generic enclosure) that work well. I used to have them connected to my Mac Pro via FireWire 400 but eventually just started connecting via USB for some reason. I think it was because PlexTools didn’t like the FW connection, and I could also free up a PCIe slot by not using a PCIe slot for the FW ports.
Anyway, I’ve found both WL7 and WL8 to work well with my external USB CD Burner. There are too many variables to know for sure where the problem is for you.