1. Ensure that the re installation of Horizon was done with a fresh new download of the software to ensure no corruptions or bugs are present.
2. Ensure that the issue persists on more then one USB device. (If this testing is possible it would be wise to try. I have seen some USB's not work and some USB's work fine)
3. Ensure that the process you are following is intended to be the correct one. (This seems like a dumb suggestion but sometimes you wouldn't believe how many times i thought i had to do something when i never did or vise versa).
4. Try and mod another game/profile and see if modding another game or instance works fine as it may be a coding issue with that specific tool you used to mod the file.
Also just give me a quick run down of what game, what tools and what procedure you follow so i can try and replicate it and see if i receive the same issue.
The possible bad end game solution is that what we are trying to do is specifically broken.