When importing the following RAW file (https://www.dropbox.com/s/5jg9ah17dxx...) in Lightroom 5 the lower right quadrant is just white, this even happens when converting to dng. Other apps can show the image just correct, so the file is not corrupt.
From your description, it sounds like the camera-embedded jpg preview is ok but the raw data that comes afterwards has become truncated or otherwise corrupted.
The applications that appear to be working aren't bothering to decode the raw data like LR is, and are just showing the embedded JPG preview. Even Canon's DPP that came with the camera does this until you tell DPP to save a JPG from the raw file, then you'll see the issue, or it'll return an error.
You can confirm this is not something LR is DOING TO your file by opening up the raw file in a program like RawDigger and seeing the same issue:
The important question is how the raw file become corrupted. Was it a flaw in the card or the transfer or the storage on your computer. Each time you offload images, I'd suggest making two distinct transfers of images from the card to the computer's hard drive--using Explorer or Finder and then do an Add import, so when one copy shows an issue, you can check the second copy and see if it also shows the same issue. If not, then the transfer, computer-hard-drive or other computer hardware are at fault. If both show the same problem then the card or the camera are probably at fault, and you can easily just stop using the one card and see if the issue goes away--if so then it was the card, itself, going bad.
You don't have to keep doing the two copies forever, just until you get an idea what is happening.