My first thought is to make sure there is no other content, or at least not the Senty folder per the instructions on Github. Also the folder name and files names are case sensitive so verify they match. Also make sure its formatted as FAT (FAT32???) And not NTFS.I'm lost at this point. Any ideas?
My second thought is did you wait long enough? It may take 60-120 seconds after inserting the drive for it to register.
My third thought would be to reformat the drive, remove your senty thumbdrive, and see if it works for setry mode with the in car format feature. When it does (presumably), delete the sentry folder and add "LightShow" folder.