I'm having the same problem. If I create a file, or download a file in a different location, then move it to the external storage, then I can modify it, delete it, or change its name with no issues. If, however, I create the file on that drive, or download a file straight from the internet to there, I can make no changes to it. I get an error stating that I need permission from "Unix USer\root" to make changes to the file. I don't even have any computers running Unix.
I have Secure Folder Access turned off, so all users should have full rights to every folder. I've tried turning on Secure Access, and creating an account with full access, but that account has no access the make changes either.