There's an interesting tool to check flashdrives.
Either "badblocks" that'is already installed or else "f3" which you'll need to install yourself.
They harken back to ye old days of using FAT with windows and fragmentation checking., except that on a flashdrive a badblock cannot be repaired. 
Here's a description/how-to: https://www.cyberciti.biz/faq/linux-check-the-physical-health-of-a-usb-stick-flash-drive/
