5diii Dual Card Slot keeps changing on its own

TimR32225 wrote:

This is how all dual memory card Canon cameras have always worked.

When you remove one card, the camera is smart enough to know that it should record to the other card by default.

When you remove one card and then later put it back, you will need to tell the camera if you again want to write to the card you just put back. It will have chosen the other card by default when your remove the first card.

There is nothing wrong with your camera. This is a case of user error in not understanding how the camera works. If it functioned differently, I suppose folks would complain that the camera should be smart enough to write to the 2nd card if you remove the 1st one.

Sometimes you just can't win.