No reason that I can think of. Will they open in the Camera Raw filter in Photoshop CC?
They do not, no.
The images sit on an external hard drive.
I have now tried the following:
Copying the problem files in the same folder (Still problematic)
Copying the problem files to a different folder on the same hard drive (Still problematic)
Copying the problem files to the desktop (Suddenly the problem files work just fine!)
Opening the problem files using a different computer (Suddenly the problem files work just fine! Although on the second computer I was using Bridge CS6).
Updating the software & rebooting (didn't help).
Another note, one of the problem files is one that I had previously opened and edited in Camera Raw a day or two before, and now I cannot open it in Camera Raw.
I'm going to guess that there is a problem that started only a day or two ago with the way the software is reading the hard drive, but I don't know how to tell for sure.
Are the problem files readonly and do they have XMP sidecars?
I should add that JPGs edited with ACR usually need to be writable for ACR to be able to store the adjustment-settings inside themselves. RAW files typically use an XMP sidecar to store settings, but JPGs typically don't.
If the files weren't read/write then ACR would have no way to store the adjustments in them and so is smart enough to not make them available for editing.
The other hint is that if you copy the files to your desktop--where the permissions are likely to be more friendly, they work.
To check the permissions, right-click on one of the files in Explorer, select Properties from the context menu and look at the Security tab. Look at the various user-groups, like Users, Authenticated Users, Administrators, etc, and see what access each has to one oft he problem files, and then to one of the files that works ok.
You might also check the Effective Rights of a particular user to a particular file by clicking on the Advanced button in the Security tab. Here is an example where I show only Read + Read & Execute access to an NEF for general users, but when I check my own logged in User (steve) I have Full Control which includes all the other access privileges, and this is either because I'm in the Administrators group or I am the owner, I'm not sure. It is possible that Bridge or Photoshop is running in a user context other than your logged in user, or it's possible that another user accessed the files and changed the permissions to them.
None of the files are read-only, and there are no abnormalities or differences in permissions. I do not know how to check sidecar data. I went into the File Info, Advanced tab, and saw there is are couple XMP sections, but everything seemed the same there regardless of whether I checked a problem or normal file.
The one file I did manage to open and edit before this problem still retains the look I gave it in Raw, though the small icons (image below) that should indicate what changes were applied are not there anymore.
If the changes are showing in the image, but the crop and settings-changed badges aren't there, then are you looking at saved copies of the original image, which has the changes baked in?
Do the images come back to life if you copy them from the external drive, where they don't work, to the internal drive where they do, and then back to another location on the external drive?
That's the weird part, I was indeed looking at the original.
Copying the files to the desktop, eg, and then back to the external seems to resolve the issue as well. I just have no idea what's going on with those problem files though.
Do you mean the badges return after copying external-internal-external? I still think it's some sort of permissions issue. LR may be running at a lower user-level than your logged in user, and so looking at the effective permissions of your own user may not indicate LR's permission level.
If you still have any bad files, a way to test this is to right-click on the LR shortcut and choose run as Administrator and see if the files work, then.