Try switching to a smaller USB drive (8GB is optimal) and use a USB 2.0 port instead of a USB 3.0 port. Older television bootloaders read USB 2.0 protocols more reliably.
: Firmware updates often include better calibration for HDR and local dimming, significantly improving picture quality for movies and games. Firmware TCL 50EP660
Navigate to (gear icon) > Device Preferences > About . Try switching to a smaller USB drive (8GB