mental ray does not support LZW compressed .tif files.
If you create a normal map using the Transfer Maps tool and select tif as your file format, Maya will create the .tif file with a LZW compression. mental ray does not support the use of LZW compressed .tif files. and an error occurs as follows:
// Error: (Mayatomr.Output) : while defining texture "file1:tex": failed to open texture file C:/temp/sampledNormals.tif
Choose a different file format for your normal map. The EXR file format is recommended. If you encounter this error, you must also restart Maya to render with mental ray again.