I haven't shot many photos with the 6DII since purchase in late November, somewhere around 100 or so "play" shots. Today I went for a walk with the dogs and a long lens and came into the house, inserted sd card in reader to import into LR - and no files were detected. I put the card back in camera and browsed photos, they were all there. I went to a second computer with LR, and then a third without and saw no files in LR or Windows explorer on the card. I directly connected camera to computer, no files. Put card back in camera, they were all there.
To end the story, all photos were lost. Nothing I could do to have anything recognize them but the camera itself. After formatting the card everything works as expected. The only other factoid that may be relevant, the card had been taken from a 5D4 just before the walk.
This is extremely scary. I can't trust this camera on a shoot until I get answers.