Hi
I dont know exactly what your problem is. But I can tell I you that as I been working with mental ray and May for several months now I had problems using mental ray and tiff´s textures. I would suggest you to bring that texture to photoshop and save it again in tiff format. When photoshop asks you for image compresion (this is after going to save as, and changing the format to tiff), select "NONE". Maya and mental ray doesn´t have a 100% compatibility with the tiff images. So try that, and if it keeps happening, just try to change the image format (maybe to a png-24) or something.

hope it helps....

JuanM