I have had a unique problem. I shoot in raw and use PSE 12 mostly but do have LR and DxO Pro as well to help PP. Here is the problem.
I switched from Windows to iMac about a year ago and found it hard to get used to for my photography but lately I have put a fire under my butt and have been doing a lot of shooting. I downloaded a card to the PHOTOS on iMAC and then tried to drag photos to the EXT HDD and they bounced back but now are THUMBNAILS I am not sure that the bouncing back had anything to do with it. Turns out the ext HDD was formatted for Windows and not Apple so that is why it was read only and wonder if it had to do with the Thumbnails happening.
The good news is that the iMAC was connected to the lap top which is a Mac Pro through the cloud so the originals are saved on laptop but this has occurred twice now.
I have had the ext HDD cleaned, formatted and then photos put back on so I have those - but the ones I was trying to save and bounced did not save to HDD and saved back to the computer as thumbnails.
HELP - Now afraid to try to save any photos to the computer.