If you can recover from the drive fine but cannot store new files, I would look at the app itself. There is a lot of issues with people using Firefox and Video download helper. Apparently with a lot of the new security updates in Firefox that app is not working for most users. If you post the app you are using we can possibly provide a better answer.
Another way you can tell if it’s the app is to try and move any other type of file manually to the drive. If that does not work then it may be a drive issue. If the file moves over fine then it may be the app trying to store directly to the drive in question.