df -h
Output may look something like this:
Filesystem Size Used Avail Use% Mounted on
/dev/sda 1.9G 17M 1.9G 1% /media/pi/USB
sudo umount /dev/sda
vFat filesystem:
sudo mkfs.vfat /dev/sda
NTFS filesystem:
sudo mkfs.ntfs /dev/sda
EXT4 filesystem:
sudo mkfs.ext4 /dev/sda