U盘Driver Corrupted, Check U盘

If it came with your U 盘 and see if there is any utility repair method. Otherwise, check the vendor's tech support website.

Or, you can save your U 盘 data on PC and reformat U 盘 under Apple.

Note, when you unplug your U 盘 from Apple you should move the U 盘 icon to Trash or highlight it under Finder and select Eject to take it out.