Which version of Lightroom are you using?
Also what is your Operating system?
Try opening images that were not exported/Processed from Lightroom..Do you still see Horizontal lines?
I am using LR6, windows, and it is only happening on the photos that were edited in lr and exported to the usb drives. I've tried different drives. When exported to my hard drive they are fine. Also, unedited photos put onto the usb are fine.
That means your USB thumb drive have the issue.Either Replace or get your USB device and driver checked