In my situation, the "R/W" card when inserted into the Canon became a "R" card because the write protect tab slid into the card-lock position.
The cause was the write protect tab was very loose and the minimal pressure of the write protect read switch within the camera was enough to slide it about 90%, resulting in a "card locked!" warning.
My solution was to glue the tab in the read-write unlocked position, which is likely a good solution for most people, as I suspect most people never choose to write protect a memory card.
Further, not every SC-card device has the ability to read the write protect request, and therefore will ignore it anyway.