The general solution for Edit In opening PS but not opening the images—from wherever the image was stored, was to reinstall PS CC 2014. Now Adobe’s solution is to install the 2014.1 update:
Since you’re only having the problem from an external drive maybe there is a different issue, but try reinstalling in case that fixes something. I think there may be some time limit for how long it takes PS to open and maybe if the image is on an external drive it may take longer. There also might be some permissions issue because it’s an external drive. You might try copying the image from the external drive to your internal drive and do the Edit In just to make sure it’s not the image, itself, rather than its location.
I isolated the issue a bit further. After playing around with a ton of photos, moving them between internal and external drives, it seems i'm only having the problem with my Canon 6D body. The photos associated with my T3i (that are part of the same album) load fine, but when choosing a photo shot with my 6D it is not loading.
Can you open the 6D photos in PS, itself, using PS / Open?
Are you using LR 5.6 and PS-CC-2014 with ACR 8.6 or older versions of either LR or ACR?
just attempted that - got an error message saying that photoshop cannot open the file, and it's telling me that i may not have the latest camera raw update. So basically that means my current version of photoshop currently doesn't support 6D raw files? (which is odd, I was under the assumption that since it's a .CR2 file it wouldn't matter which body took the photos, but I guess that's not the case).
So if I update it then that should be it?
Every camera model has to be profiled and programmed for specifically since the sensors are generally different as that is a key thing the manufacturers are working on to improve, so if your version of software is older than the camera then the software won't know how to deal with the raw data.
You haven't answered the question what versions of what software are you using? You can use Help / About Photoshop or Help / About Lightroom or Help / About Plug-ins / Camera Raw or About Plug-ins / Camera Raw depending on what version of what software you're trying to determine the version of.
The other thing to know is that Adobe doesn't update older versions of their software for newer cameras--they only update whatever the current selling version is at the time, so if you have, for example, PS-CS5 or Lightroom 4.x then those obsolete software versions would never be updated to handle cameras that were new to market, this year, since the current versions of software are LR 5.x, PS-CS6 and PS-CC..