This only happens when I use that one particular lens. I only have that one Tamaron lens as the rest are Canon.

The only time I have seen this happen is in the following settings:
RAW + JPEG
Parameter 1
Using any Creative Zone

This morning I finally found a way to stop it from happening.
Turn the ISO expansion ON in the custom functions.

Quote - Yes have to agree with you about the extra space usage being a pain. I always shoot in raw/jpeg. But trying to remember when i did the B&W parameter if i ended up with four photos. Thought just two. Will try tomorrow to see what i come up with. I just use Canon lenses and also have Canon20D.Have a great weekend:)
Chris