All flash media cards should be treated as unreliable (source: 10 years photographing professionally). An errant static discharge, a damaged pin, or a cracked piece of thin plastic are all that stand between an easy offload of photos and possibly losing them completely. I _always_ configured the cameras to mirror to both card slots in order to have an instant backup, placing a larger capacity SD card in its slot and using smaller CF cards as the primary and swapping in and out. In my experience, CF cards are more durable due to being thicker, so I was more comfortable swapping those in and out regularly.