Connect your USB disk to your computer and run this command to know where it is mounted
sudo fdisk -l
To be able to format it, you need to unmount it using this command
sudo umount /dev/sdc1
You can format your USB disk using mkfs
# FAT sudo mkfs.vfat -n 'device name' -I /dev/sdc1 # NTFS sudo mkfs.ntfs -I /dev/sdc1 # EXT4 sudo mkfs.ext4 -n -I /dev/sdc1
You can also use following commands to completely wipe out the whole USB flash drive
# use shred sudo shred /dev/sdc1 # or dd sudo dd if=/dev/zero of=/dev/sdc1