Let me add to the confusion. I usually shoot my 1DIV in JPG, just in case I need the buffer depth. I mostly use it for athletics/sports. I almost always take the card out and use my PC's card-reader (CF & SD both built-in). I assume my answer above was based solely on JPG files.
After reading TS's post above, I went menu diving for something like the 5Dc's PTP setting, which messes up downloads. I didn't find it.
What I did discover is really weird. Let me state that I did use two different USB cables and got the same result with both. PC1 is a Windows 7 Dell desktop. PC2 is a Windows 8 HP laptop. Both computers are running LR 5.7.
PC1:
1DIV raw files on either card are not visible in LR Import dialog when using USB cable. If I take out the card and plug in into my PC's slot (have CF & SD), ALL files are visible and importable.
6D raw files show are visible/importable in LR Import dialog using USB cable or built-in card reader.
PC2:
Raw files form both the 1DIV and 6D are visible and importable in LR when using a USB connection to the camera. My laptop does not have a CF card reader, but apparently does not need it. The USB connection works fine with either camera.
PC1 & PC2:
All JPG files are visible/importable using a USB connection.
Just for fun, I shot my G15 raw. JPG and cr2 files are visible/importable in LR when using a USB cable, on either PC.
Conclusion:
I apologize for the confusion from my first post in this thread. The 1DIV does seem to have a disagreement with Windows 7, with raw files.