I am able to open some but not all of my .CR2 files with Camera Raw 6.7 and Photoshop CS5. On my Canon Rebel XSi, if my Quality setting is set only to RAW, the .CR2 files open in Camera Raw 6.7 and Photoshop CS5 with no problem. If the Quality setting is set to RAW and L (large jpg), those .CR2 files DO NOT open with Camera Raw or Photoshop. I have installed various versions of Adobe DNG converter with no success. Canon's Digital Photo Professional and RawTherapee would show thumbnails but could not convert, 'save as' or batch any of the suspect .CR2 files. My mac's OS is 10.6.8. Thanks in advance to anyone who takes time to offer a solution.
Check the version of DPP installed on your computer. The DPP version on your computer should be the one that is compatible with your camera. Higher version is not necessarily the best for your camera.
I faced the same issue as you had. My camera was 550D and the DPP I had installed on my computer was v3.5.x (which was downloaded off the Internet because I misplaced my CD). All images shot with (RAW+L) showed the CR2 images as X.
I downloaded the right version of DPP from Canon Europe site (which was DPP v3.14.48) and all my CR2 files were able to open and edit without any issue.
So check your compatible DPP version from http://www.canon-europe.com/support/consumer_products/product_ranges/cameras/eos/ . Select your camera and download the right update for your DPP.
Try using DPP with your 'unopenable' raw files. If DPP doesn't open them then there may be something wrong with your camera or memory card.
Have you tried using a different memory card? have you tried shooting tethered w/o a memory card and have the images copied straight to the computer?