What helped me: recreating the vfatvfat filesystem on the USB will format the disk and delete all the files:
umount /mount_pointumount /mount_point (e.g. umount /mntumount /mnt)
mkfs.vfat /name_of_the_partitionmkfs.vfat /name_of_the_partition (e.g. mkfs.vfat /dev/sdc1mkfs.vfat /dev/sdc1 - make sure to use the correct partition to not delete data from other disks)
mount /name_of_partition /mount_pointmount /name_of_partition /mount_point (e.g. mount /dev/sdc1 /mntmount /dev/sdc1 /mnt)
cd /mount_pointcd /mount_point
llll (to check you are in the correct partition and there should be no files now)
