Hi Greg,
If you're running Mac OS 10.4+, then Photo Mechanic had used the RAW data as the source for the JPEG file. You will need to disable Photo Mechanic's RAW Rendering so that the embedded JPEG preview will be used instead. This setting is found in Photo Mechanic's Preferences under the RAW tab. Simply uncheck the box to "Enable RAW Rendering (using Apple's ImageIO)" and the B&W preview will now be the source.
Regards,
Jerry