My 1D3 will not change over media automatically. For starters, I have checked and double checked that I have the menu option "Record Function + Media/Folder Select" set to "Auto Switch Media"
The first time I was shooting a soccer match and as the game was ending I noticed the CF card counting down into the single digits. I shot perhaps another 20 shots and I was really surprised to get home and find -zero- images on the SD card. I had a feeling the last few shots I took were missing but I figured I must have been mistaken.
Then this week I caught it in the act. The CF card was filling up and I watched it count down to zero. On the next shot I looked at the camera to make sure it had switched over to the SD card. It didn't, and instead the LDC was telling me "CF card full" and the shots I was taken were simply not being written to anything. I hit the 'Function' button and switched manually to the SD card and it worked fine from there.
Any ideas what I'm missing here?

