- $ sudo umount /dev/sdX (if the USB stick has been mounted; X = a, ..., z)
- $ sudo mkdosfs /dev/sdX
- $ sudo su -c "zcat boot.img.gz > /dev/sdX"
- $ sudo mount /dev/sdX /media/mountPoint
- $ cp *.iso /media/mountPoint
Before the above steps, the capacity of my 8G USB stick was
After the above steps, it became only several MB
But I don't know what caused this.