LG tell you this:
"
Connect the USB storage device to the [USB input] port on the back of the TV.
Use a USB memory of 32 GB or less or an external storage device of 2 TB or less.
Storage devices with too much capacity may not work smoothly."
LG TV - How to Connect a USB Storage Device to Your TV. Learn how to use, update, maintain and troubleshoot your LG devices and appliances.
www.lg.com
You don't say what capacity device you're attempting to use.